This is a short overview of the Tyranid units from my experience, including suggested uses and upgrades for the squads that the Tyranids have at their disposal.
Ripper Swarm
The Ripper Swarm is one of the cheapest units in the game, costs nothing to reinforce and reinforces instantly. This makes it an excelent unit to use for capping and disruption, it's melee attack forces units that fight it to melee back, this is extremely useful against set-up weapon teams as they can no longer fire their main weapon when fighting the Ripper Swarm. Although it is also useful against enemy ranged units such as guardian squads, tactical marines and warrior broods with ranged upgrades. These little guys make excelent capping units as they cost nothing to reinforce and are extremely durable against ranged fire, they have almost no damage which means they aren't viable as anything other than disruption, except against very weak melee units such as guardians and scouts, but when under the effects of synapse they become immune to supression, making them ideal for setting up an assault against an entrenched position. I have rarely found a situation where more than two Ripper Swarms are useful but I would advise getting at least one in every game.
Hormagaunt Brood
These little guys make for great starting units, they're quick and they're deadly in melee combat against weak squads such as guardians, scouts and set up weapons (who are terrible melee units). Their speed allows them to out-cap your opponent and to cover the distance to their target extremely quickly and with minimal damage. Their high dps allows them to quickly overcome any soft target, in addition to this the Hormagaunts are cheap to reinforce at fifteen a member. However the low life of the individual members of the squad make them extremely weak against AOE weaponry and a poor choice beyond the early game. With the melee synapse the Hormagants gain damage and begin leaping over cover and previously impassable terrain, this is extremely good against strong ranged units such as guardians and set up weapons that position themselves behind cover as you can engage them much quicker. Hormagaunts also gain extra life when the Hive is upgraded a tier.
Termagant Brood
Termagants are effectively the ranged equivalent of the Hormagaunt, they're just as quick as Hormagaunts making them ideal for kiting enemy units but their DPS is significantly less than that of the Hormagaunt. They're a good choice against the Orkz because the Ork Slugga Boy will tear through Hormagaunts but Termagants are quick enough to kite them. The real Benefit of the Termagant is the fact that anything struck by a Termagant's weapon becomes more susceptible to other incoming damage, making them a must-have support unit in many engagements. Under the effects of ranged synapse they also gain a chance to cause knock down with their attacks, which anyone who has faced shotguns will know is extremely powerful. Termagants also gain extra life when the Hive is upgraded a tier.
Warrior Brood
These guys are the bread and butter of the Tyranid forces, capable of upgrading to deal with any threat and strong enough to withstand a lot more punishment than other Tyranid units, the Warrior Brood is really your main unit. Warrior Brood lack the speed of the smaller Tyranid units but easily make up for it in sheer brawn, they field an impressive amount of HP and a strong melee attack as well as projecting a durability synapse aura they can also be upgraded with one of three weapon upgrades. The Barbed Strangler equips a single member of the Warrior Brood with a slow firing ranged weapon that causes instant supression to units caught in it's area of effect, this can be extremely strong against horded units such as Guardians, Orkz or Tyranids. The Venom Cannon equips one of the Warrior Brood with a powerful ranged weapon capable of destroying vehicles, this is extremely useful against kiting vehicles such as fast Razorbacks or Falcons. If you want more ranged support, it also provides the ranged synapse which greatly improves ranged units, especially Termagants. The final weapon upgrade is my personal favourite and the one I would recommend getting in almost all situations, the Adrenal Glands upgrade improves the melee damage of the entire Warrior Brood as well as granting melee synapse to nearby units, the true glory of the Adrenal Glands however is that it makes the Warrior Brood capable of severely damaging vehicles, so much so that two Warriror Brood can take down almost any vehicle in short order.
Spore Mines
One of the deadliest units currently in the game, although capable of a weak melee attack that can technically tie up a unit the real power of the spore mines comes from their Detonate ability, which instantly kills the Spore Mines but deals a large amount of damage in an area around each mine. The damage is great enough to kill any weak squad in one explosion and the area of effect allows it to completely devastate clumped up units. For 100 Requisition the Spore Mines are easily capable of dealing their own cost in damage many times over, but the cost of fifteen power can significantly slow the teching of Tyranids so it is a trade off the player must decide to make. Spore mines can also instantly kill the majority of any garrison within a building they detonate near making them the Tyranids most effective tool against garrisoned units. Spore Mines are fairly fragile and die quickly to focus fire making the delivery of them extremely important, this is aided by the use of Tunnels which can quickly get spore mines into a position where they can deal maximum damage.
Lictor
The Lictor is a solitary assassin unit, the fact that the entire squad is comprised of one unit makes preservation of the Lictor extremely important. The Lictor is capable of Infiltrating, making it invisible to all but detector units making it an excelent choice for disrupting entrenched positions before an assault, it is also capable of using the Meat Hook ability which fires hooks into a single target and drags it towards the lictor, dealing damage in the process. This is exceptionally useful used against Hero units as it allows you to pull them from the safety of their army and swarm them before your opponent can react, it is also very poweful against Space Marine players, as their squads are so small in size that the instant loss of even one member (especially the sergeant) can drastically reduce their fighting capability. Meat Hooks can also be used to pull the gunner of a set up team, giving you time to swarm the remaining members or to pull men from garrisoned structures. The Lictor is also capable of peforming an Assault Jump when upgraded with Vanguard which also increases the health and damage of the Lictor. This is similar to the Assault Space Marine and Stormboyz jumping ability but is less devastating due to the Lictor being a solitary unit, it is still extremely useful for disruption however as it still causes knockdown to units in the targeted area. The Lictor is also capable of detecting invisible units.
Zoanthrope
The Zoanthrope is the Artillery unit of the Tyranid army, it is fragile but has a Warp Field ability that creates a shield that causes attacks to remove his energy not his life, although it stops energy from regenerating whilst active. The Zoanthrope is also capable of a strong attack that can hurt vehicles. The Zoanthrope's strongest ability is the Focused Warp Blast that deals damage and disables vehicles for a period of time. This is extremely effective against fast vehicles that can kite your Warrior Brood and against Turrets, since the Zoanthrope can outrange them. The Zoanthrope's main use in 1v1 is it's ability to return fire on harassing vehicles and it's health regeneration Synapse, although it can also provide decent fire support against infantry.
Carnifex
The Carinfex is a huge monster capable of wrecking havoc upon it's foes. The carnifex has relatively low life but a strong armour that significantly reduces most attacks against it. It is also capable of dealing huge damage to anything it gets close to. The Carnifex is capable of three upgrades to improve itself. The Thornback upgrade is the most common, despite being arguably the weakest, as it increases the life of the Carnifex and unlocks the mighty Charge ability that allows the Carnifex to accelerate greatly as it charges towards it's target, knocking down infantry that get in it's way. This can be stopped by a number of obstacles however. The second upgrade is the Barbed Strangler and Spore Cysts upgrade, which in my opinion is the best because it grants a Barbed Strangler that supresses all infantry that it hits. It also allows the deployment of Spore Mines from the Carnifex for no cost and a relatively short cooldown. The major weakness of this upgrade is it's significant reduction to the canifex's combat power meaning it is only an effective choice when you do not need your carnifex for anti-vehicle duties. The final upgrade is the Venom Cannon which is a ranged weapon that is extremely effective against vehicles, this is a close second to the Barbed Strangler and Spore Cysts. It can even be a better choice when direct damage output is your primary concern (over time the spore mines will give you far more effect but this upgrade is more useful for direct combat, with the buff to plasma weaponry it is also far stronger than it used to be). In addition to this it grants the Bio Plasma ability which fires a ball of Bio Plasma that does significant damage to all infantry in the targeted area. The Carnifex is capable of being upgraded to suit any situation but it's large cost and relatively low life, combined with lack of agility, make it a good choice only if it can be properly support.
Ravener Brood
The Ravener Brood is a group of three Raveners. Although relatively fragile they can deal huge amounts of damage to their targets in melee combat. They are also capable of burrowing into the ground, making them invisible to all but detector units but still able to move freely. When they emerge they cause knockback to nearby units making them excelent suprise troops to turn the tide of a battle. They can be upgraded with the Devourer, which grants ranged weaponry to the brood, similar to the weapon of the Ravener Alpha. The Devourer does good damage and is effective against all targets, although it's damage is limited at best against vehicles. The Ravener Brood also has the Fleet of Claw ability that greatly increases speed at the cost of energy.
Discuss this guide in the Tyranid Unit Overview discussion thread.