General Army Notes:
The Orc and Goblin army has the greatest selection of troops available. While their average stats and abilities may not seem like much when taking each unit individually, the power of combined units of Orcs and Goblins is not something to underestimate! A combined force of multiple units in an Orc/Goblin army is definitely something you should strive for.
Foot Units:
Orc Warriors: You must take two mandatory units. Orc Warriors are definitely the core units in your army, and though their average wounds and unimpressive save means they are slightly vulnerable, their high number of attacks means that even the two mandatory units can cause some massive damage in an assault.
Black Orcs: Black Orcs are your main assault unit, able to deal out damage and take it. Their relatively high cost and low number of units means that you will not be able to take many, so make sure to back them up with a unit or two of Orc Warriors.
Goblins: You must take two mandatory units. Goblins may seem at first to be the perfect cannon fodder, but their cheap cost means you can get quite a few units, and their numbers may make the difference between a victory or a defeat in an assault. They are also useful in their role as missile units.
Trolls: Regenerate 1 wound per base per round of fighting (cannot bring back dead bases). Trolls are Stupid and suffer a -1 to all order rolls. You can take up to three units per 1000 points. Trolls can do some decent damage in an assault and might stick around for a while, but due to their special rules they are rather hard to get into an assault.
Ogres: Ogres must charge humans on initiative. The charging humans special rule makes controlling Ogres a bit of a hassle, as a crafty opponent may use different human units to draw Ogres into an assault on their own turns. Be very cautious of this, as Ogres should not be wasted in a useless assault.
Cavalry, Chariot, and Monster Units:
Boar Riders: Boar Riders make an excellent assault unit, being fast and striking hard in an assault. Support in the form of Chariots (assaulting) and Wolf Riders (supporting fire) will make these units even more deadly than they already are.
Wolf Riders: Wolf Riders make a good support unit, either supporting Boar Riders with their missile fire or moving about the battlefield quickly and lending missile fire where needed. Due to their relatively low cost (cheaper than some infantry units!), Wolf Rider units can be taken in multiples, and probably should be.
Wolf Chariot: Wolf Chariots are excellent for supporting an assault or charging weak units in the open, but do not expect them to do well against harder melee units.
Giant: Causes Terror. Madness special rules - if you fail an order test with a Giant, you must roll on the Giant Madness Chart (which would cause some damage to your own troops). One unit per 1000 points. Giants are nasty in an assault, and are very survivable, but be careful in their deployment and try to support them if possible. Even with their ability to do massive amounts of damage, however, there are better things to spend 150 points on.
War Machines:
Rock Lobba: Cannot stand and shoot. Rock Lobbas give you some good long-ranged supporting fire, but be sure to give them something to survive assaults by flying or fast units.
Characters:
Orc General: The Orc General's low Ld means that he has a higher chance of failing a test for giving orders than other generals. An Orc General should not be thrown away in an assault, even with his +2A. Remember, he is your general and one of your few characters, and you need all the characters you can take in an Orc and Goblin army.
Orc Heroes: Two max per 1000 points. Orc Heroes can be very useful lending their weight in an assault, but their Ld is too valuable to throw them away blindly assaulting enemy units. Remember that your General's low Ld means that the Heroes often have to give out a lot of orders themselves.
Orc Shaman: The Orc Shaman is pretty much only useful if you want to have some spellcasting abilities in your army (and the ability to dispel your opponent's spells).
Goblin Heroes: Two max per 1000 points. Goblin Heroes make great leaders for Goblin units, where their extra attack is needed. However, even with their low Ld, they can still give orders if needed, and this ability should NOT be overlooked.
Goblin Shaman: The Goblin Shaman gives you a cheap spellcaster, which can be useful at times even with his extremely poor Ld. The 45 points is worth it to have spells of your own and the ability to dispel your opponent's spells.
Wyrven Mount: The Wyvern's high points cost may seem like a bit much, but when you stick a cheap Goblin Hero on it you'll have a good supporting unit that can move up to 100cm from unit to unit and give its weight where need, for a pretty decent cost of only 125 points. Don't cut down on your units to take this combination, however, as more units can at times be much more useful than flying support.
Chariot Mount: The Chariot gives a character a little bit of extra hitting power and some mobility of its own. Don't count it out as a possible mount.
Special thanks to Dalkwave for his help on this tactical overview!