There are so many different build orders to use against each race it's ridiculous. Also consider that good players will be able to shift their build to better react to their opponents.
That being said, I think the most common one is some variation of a 9 pool build.
* 9 - Spawning Pool
* 8 - Drone
* 9 - Extractor
* 8 - Drone
* 9 - Cancel Extractor
* 10 - Overlord
* @100% Spawning Pool - 6 Zerglings
I should warn you that I am no SC expert player, or anything. I get most of my info from http://wiki.teamliquid.net/starcraft/Strategy_%28Zerg%29
Knowing build orders, knowing when to use a build order and knowing when to change your build order on the fly are all very complicated questions with very complicated answers. If I were you, though, I would start with the 9 pool. Learn it's advantages and weaknesses over other builds. Also you should think about what the build is capable of, what you are able to tech into from this build and etc.
Traditional unit Tiers were based on the Warcraft 2 model where you upgraded your main worker producing structure to get new units. Since both races had 2 possible upgrades, this lead to 3 tiers.
Starcraft 1 mimic'd this with the Zerg race but not all races. However, the terminology had stuck for how to describe various phases of the game. Tiers referred to how deep into the tech tree a player had gone and how much resources/time they had spent on tech. As a result a general terminology emerged, placing most units in a tier.
With the advent of Starcraft 2 and the addition of a number of earlier units, there has been a rise in referring to certain units as tier 1.5. These are units after tier 1, but still before tier 2.
Regular terminology is detailed as follows:
Tier 1
Tier 1.5
- Roach, Baneling
- Marauder, Reaper
- Stalker, Sentry
Tier 2
- Hydralisk, Infestor, Mutalisk, Corruptor, Overseer
- Ghost, Hellion, Siege Tanks
- Immortal, Phoenix, Void Ray, Observer, Warp Prism
Tier 2.5 (sometimes referred to as tier 2 or 3)
- Banshee, Raven, Viking, Medivac
Tier 3
- Ultralisk, Broodlord
- Battlecruiser, Thor
- Colossus, Mothership, Carrier, High Templar, Dark Templar
You'll note that tier 2.5 has only terran units, and is often just referred to as tier 2. This is true for all of 2.5 except for the Raven. Because the Raven requires so many upgrades to be effective, its sometimes referred to as Tier 3 and others as Tier 2. Its also worth noting there is some discrepancy around which tier ghosts should belong to.
These naming conventions are not absolute, but are common amongst professional casters (people who comment on professional games).
Best Answer
Here's a summary of what it does:
"Doing the Eraser trick is very easy, you only need two or more Science Vessels with Irradiate researched. To do the trick, simply have your two Science Vessels irradiate each other, and fly them over or near the units you wish to kill. Since Science Vessels are not biological units, Irradiate has no effect on them; however, Irradiate's splash damage hurts surrounding biological units."
taken from http://wiki.teamliquid.net/starcraft/Science_Vessel_Eraser