There are currently five multiplayer armies:  Space Marines, Orks, Tyranids, Eldar, and Chaos Space Marines.  Each army comes with three different commanders, meaning three different sets of tactics to use with each army.  Once you choose an army and a commander, it's off to the battlefield!

Resources are gained directly on the battlefield in the following way:

Requesition:  Gained from Requesition Points.  The longer you hold a point, the more Requesition is generated per minute.  Requesition is used for buying new units or upgrading existing units.

Power:  Generated from Power Nodes.  If you capture a Power Node, you can upgrade it with Requesition.  After upgraded, you can add up to three generators to further increase its power output.  Power is used for upgrades or specialized units.

Red:  The official name for this resource varies from army to army.  You gain it when your units die or when you kill an enemy unit.  (NOTE:  Orks are the only army able to generate Red passively)  Red is used to activate special global abilities.

 

Now that you know the basics, we'll get into the bare bones of each of the multiplayer armies.

Space Marines:  Easily the strongest infantry and vehicles who can lick their weight in Xenos.  Shaky when it comes to numbers, they have the most expensive units, but they also usually suffer the least in casualties.

Eldar:  Comparably fragile units, who are among the best suited for ambushes.  Their high speed and damage make them deadly in the hands of a master, but they're usually recommended for advanced players.

Tyranids:  A mess of unit types that range from the weakest of the weak to immune against small-arms fire.  This is the most number-heavy race, dedicated to swarming over their foes in a tide of fangs and barbs.

Orks:  The second most number heavy race in DoWII's multiplayer, Orks are the "horde" race.  Their units are comparably fragile when compared to others, but all Orkish infantry has the WAAAGH! Shout, an ability that improves their stats for every time it stacks.

Chaos Space Marines:  Similar to the Space Marines and Orks, but with a few technical differences.  Not as sturdy as the Space Marines, but not as fragile as the Orks, still bearing several support abilities to improve their performance.

Hopefully this helps you choose an army for your multiplayer campaign!  Good luck!