The Winter Assault Order Campaign consists of five missions. In the first three you control both Imperial Guard and Eldar forces for predetermined parts, in the fourth you can switch your faction at will and in the fifth you control the faction that won mission four. You will usually face Disorder (Chaos Space Marine and Ork) forces, but also Necrons in mission 5 and you may have to fight the other Order faction a bit in missions 4-5.
If you are not already familiar with playing the Imperial Guard and Eldar factions from multiplayer games, you should play the tutorial games (choose Skirmish at the main menu and Tutorial at the Skirmish menu) for both factions to get familiar with their basic units, structures, technology tree and strengths; this really pays off, even though it may seem tedious. Unless you feel very confident (being a Dawn of War or general RTS veteran already) you should of course choose the Normal difficulty level.