Stratagist IV
Ex virtute honos
Hello, I was just wondering (since I am new to civ 4) what do all the promotions do, I keep forgeting 

Go here ==> http://www.civfanatics.com/civ4/info/promotions/
That will give you all the information about the various promtions available as well as what units can obtain said promotions.
It's not as straight forward as you'd think...Just remember, the combat promotions provide bonuses to the unit that earned them. All others (City Raider, Cover, ect.) provide penalties to the unit your unit fights against should the penalty apply.
With the following promotions available to units joined to a warlord.
From: http://www.civfanatics.com/civ4/warlords/#warlord
Combat VI: +25% strength
Leadership: +50% experience from combat
Medic III: Heals units in same and adjacent tiles extra 15% damage per turn
Morale: +1 movement
Tactics: +30% withdrawal chance
For a full discussion, go here. http://forums.civfanatics.com/showthread.php?t=137615
It totally depends on your situation. You need to know what the unit is going to do to know what promotions to give. If you don't know, combat 1, 2, 3, etc. is fine. Some examples:
Planning a chariot rush on enemy? City on a hill with culture defense? It's going to take a lot of chariots. To reduce your losses, promote at least as many chariots with flanking 1 and 2 as there are archers in the city. You want to use flanking for units that are the cannon fodder for the initial assault to soften up defenders as they will usually lose, but often withdraw from combat. The rest of your chariots should get combat promotions. You may also consider upgrading one of those flanking 2 chariots to sentry to get valuable extra intelligence.
Planning an axe rush on nearby city? Obviously a lot of city raider promotions, but also include some defensive axemen. Does opponent also have a couple axemen? Promote one of your axes to combat 1, shock, combat 2, or if you are planning to invade from a forest tile, combat 1, shock, woodsman, or woodsman 1 & 2. Opponent have archers he will attack with, make sure to include a cover promoted axeman. Attacking from a hill? If you have archery, include a guerilla 1 & 2 archer. Opponent have mounted units? You will want to build a spearman to come along.
Having barbarian problems? Guerilla 1 & 2 archers fortified on a hill are excellent. Add in a drill 1, 2, 3 archer to take down the injured barb in case the guerilla archer dies. City defense for archers is great if you need it, but aside from newly built or captured cities, you don't want to be defending from cities. Get them barbs before they pillage.
Yea, I try to enhance the preset benifits of the units.If you're using Axemen to fight off barbarian attacks, it's good to load them up first with combat 1, and then shock, so they can take on the most serious barb threats - swordsmen and other axemen.
Yea, but you have to have horses.( In most of my games I don't get them until the 1800's!)Remember that in Warlords, Chariots receive a 100% bonus vs Axemen. If you have horses early build a few and you'll have zero problems when the Barbarian Axes come callin.