Unit Overview

Zealot
These nifty little guys are among the best ground units in the game (at least when their speed is upgraded.  They are an effective rush unit.  When rushing, try to get 4-6 of them, and then send them to attack your opponent.  When rushing, avoid the players defense (especially bunkers or cannons) and hit the peon line.  A group of zealots combined with cannons or dragoons is a great combination too, as the zealots will act as the frontline force while protecting the goons and cannons.  Try to combine zealots with goons or archons, cause zealots cant attack air units.

Dragoon
I used to think dragoons were the crappiest unit in the game.  Upon further study, however i have concluded this: 1 dragoon sucks ass, but Groups of dragoons kick ass!!!  Dragoons can act as good support units, as they provide early-game anti-air for zealots.  In fact, zealots and dragoons work really well together, and can rape your enemies if mass produced early in the game.  As far as using dragoons by themselves, you hafta have LOTS of them, as any number of goons under 8 by themselves really sux.  One on one, they will lose to a zealot, and be killed pretty quick by a hydralisk (and even a zergling).  Firebats are also things to watchout for, but most terran units one on one will lose and lose bad. 

Templar
Templars will save your life against masses of enemies.  At first glance, they seem to suck, as they have no "normal" attack, but this is REALLY not so.  The tool for a protoss player to destroy a good zerg player lies within the templar.  The temps have 2 spells, and both are awesome.

PSI Storm:  This spell kicks major ass, and is really the only way a toss player can withstand a beating by a good zerg player who builds masses of hydras, mutas, or guardians.  Just try to cast it in the middle of a bunch of units that are close together, or slightly in front of where the units are heading, and watch em burn!! If you upgrade your temps with the energy upgrade, then a single temp can cast PSI 3 times, meaning DEATH for all who oppose it. 

Hallucination
This spell is especially useful in those "long" games that seem to be a stalemate.  Against scourges or PSI-happy temps, make hallucinations of your attacking units (especially carriers, in the case of scourges) and send those in ahead of your "real" force.  The scourges will then kill your hallucinations, and your real carriers will be alive and ready to kick ass.  On a reaver drop late in the game, hallucinate some shuttles, and send them in with your real shuttle, and the real shuttle will most likey land safely, while the hallucinations draw fire from the enemies defense.

Archon
I love these guys; archons are my favorite unit in the game. period.  They mame ground units, and tear shit up really fast.  1 archon can kill 3 or 4 mutalisks all by himself and smile afterwards.  They are really tough too, i once had a single archon defending a choke point, and watched as he killed 3 waves of zergling/marine attacks all by himself!!  The only thing to watch out for when using archons is the EMP shockwave from terran science vessels, as they will remove all shileds from archons with a single shot.  Since 99% of archons hitpoints is shields, all your archons will basically die in a single EMP (thats bad).  So, a good rule of thumb is: when playing terran dont make archons! =)

Reaver
Reavers are very powerful, but need lots of support to be used effectively.  A good reaver drop has to be executed VERY quickly to be effective (within 10 minutes, at the very latest!!). It is possible to use reavers as defensive units (like siege tanks), but they lack the range of tanks, and also kill your friends units in an attack (although siege tanks also do).  When attacking with reavers, its good to have support (like zealots or dragoons).  Reavers are VERY slow moving, and require a shuttle (with upgraded speed, preferably) to move with any effectiveness.

Scout
On the whole, scouts are ok, but, like reavers, they are support units, and really not much better than that.  Sure, they will rape muta's and, in large numbers, battleships and carriers, but they will NEVER kill someone by themselves.  They are really only good for escorting shuttles and harrassing enemy probes and expansions.  With the upgrade, they are really fast, but still, their ground attack is pathetic, and unable to destroy a base anywhere near fast enough.

Carrier
Carriers are really a mixed bag.  With the capacity upgrade and armor and weapons upgrades, they are the best air unit in the game, as they simply destroy EVERYTHING that opposes them in a matter of seconds.  Not to mention they look really cool, with all  the interceptors flying around killing everything! =)
Now the bad news.  They are very slow, VERY expensive, require quite a long trip up the tech tree to make, and are very vulnerable to scourges, templars, and any other air-air unit.  In fact, they are so weak, that a fleet of 12 carriers will be decimated by PSI storms quite quickly, assuming your opponent has more than 3 templars with the energy upgrade.  The worst thing about carriers however, is that it takes too long to actually get them.  For a fast player who turtles and makes cannons and goes straight to carriers, it still takes around 12-15 minutes to get 4 carriers.  This is far long enough for a decent player to attack him, break through his defense, and kill him before he gets the carriers, or is able to build interceptors. 

Arbiter
Arbiters are one of those "late-game" units that have kick-ass spelss and can not stand on their own, but can be used to support units like carrier fleets.  Arbiters have 2 spells, and they are must-have's in long games.

Recall
This one is sorta confusing at first, but heres how it works.  It is used to transport a group of units within a certain radius of each other to the location to where the arbiter is.  First, move the arbiter to where you want to move the units to. Then, select "recall" and a target spot will appear.  Move the target spot to where the units are, and click on the middle of the group of units.  The arbiter will then instantly warp them to its location, and cloak them at the same time.  This is a good way to go in the "back door" of an enemies base for a suprise attack, or to move injured units to a safe location. 

Stasis Field
This kicks ass in battles vs other enemies, especially carrier vs carrier or carrier vs battlecruiser battles.  Simply move in for the attack, and then select the arbiter and stasis the enemies fleet.  Although you cant attack the enemies units while they are frozen (unlike lockdown, in my opinion the cheapest spell in the game - although PSI comes close), you can proceed to trash their town, and THEN defeat the units after they come out of stasis. 

Observers
These guys are the best recon units in the game.  Although they arent really necessary against anyone except terrans (or toss's dark templar in BW), they can tell you what the enemy is planning, if you are skilled at maneuvering them on the map.  Litter these guys around your base vs terrans to prevent against nukes.  Keep 3 scouts around to kill the ghost after you spot him, and your base will be nuke proof.