The Zerg have few weaknesses. But to counterbalance that is the sheer quantity of units they can have and everything seems to balance out nicely. They are prone to area attack specials and Irradiate. They can only heal at a steady rate (a big deal for Queens, Guardians, and Ultralisks) while other races have repair and shield battery. Their specials must be cast before opposing forces mixwith yours, thus you can't react slowly to an attack versus Zealots, etc - you must have good recon.

The Zerg's strengths are very important in any game. Zergs benefit highly from weapon upgrades (because lots of Hydra/Zergling). Zerglings benefit most from attack upgrades (along with Protoss Interceptors) since they both start out at 5 damage and go to 8 damage - that's a 60% increase in damage! Get the zergling attack rate upgrade and you basically double your damage. The Zerg expand and gather resources better than anybody. Use your second hatchery in your first town to get closer to the minerals or gas. Their hatchery is super versatile (it can produce any type of unit) and they can get a defense or offense of any type quickly by producing three or six units at a time per base.

Overlord: With the speed upgrade, these are excellent, cheap scouts. Use them to watch resources, and spot spider mines and wraiths. If someone foolishly tries to go around killing them with air units, get your scourge ready!

Drone: You can counter irradiate or parasite by changing your drone into a building. Use a drone early on as a quick scout. Remember to keep him alive if possible (marines pose the most threat). Drones do not set off spider mines, so use them to attack spotted ones if convenient. You can burrow these guys when an enemy sneaks into your base.

Zergling: Zerglings make good cheap scouts early on. Very good scouts once you research burrow. Place them throughout the map especially at resources. Always have one Zergling burrowed/scouting a bit ahead of your base to detect incoming trouble and another right in front of their main base to detect units moving out. Don't go too wild very early on with placing Zerglings as scouts because you will need them to fight. However, the more the game progresses, the more Zergling scouts you should use. These guys are your main defense for early offenders, and also your best bet at getting a rush in quick, especially useful against the Protoss. Zerglings are great building killers including towers. They are very powerful when their attack is upgraded.

Hydralisks: Better known as Hydras, are arguably the best unit in the game. They are very cheap, 75 minerals and 25 gas; they are very quick to build; they have a ranged attack with a quick fire rate that kills air and ground units well; and they move fairly quickly once upgraded for speed. Their big weaknesses are spider mines, Siege Tanks, Psi Storm, and Reavers. Hydras kill buildings and towers quickly.

With careful control, a group of Hydras can take out a similarly numbered group of Zealots (especially if the Zealots don't have the speed upgrade and you do.) The more fighters involved, the better for the Hydras. The reason? The fact that you can "walk around" the Hydras that are taking damage to the back of the Hydra group if the group is positioned correctly. Some Hydras will not be able to make it, but the majority will, and that means that the Zealots have to choose to follow the ones doing the walk around and get hit all the while by the other hydras, or start over on the Hydras that are at the new front line. If they go for the new front, they Hydras that did the walk around can start firing on the Zealots again.

Hydras can cut through Zerglings, Marines (unless the marines are in greater numbers and sometimes with stimpacks), a tad lesser numbers of Zealots, etc. They kill all things quickly, especially towns. Always upgrade the speed for hydras, and if you aren't needing queens and can afford the gas, upgrade the range too. Once you have an expansion, you definitely want to upgrade their attack damage since they fire so quickly and you should have so many of them.

To give you the idea of the power of hydras, let it be known that 4 hydras with no upgrades can kill an Ultralisk with no upgrades without much walk around around usage. At worst you'll lose a Hydra. Compare the prices: 4 hydras = 300 minerals and 100 gas and 4 supply, while an Ultralisk costs 200 minerals and 200 gas and 6 supply. What does this mean? It means that the hydras are a much better deal since they take less gas and are anti-air. The Ultralisks are better in situations that involve many Archons, Psi Storm, or terran defense.