First ill deal with the, single player, it consists of around 10 missions, 3 Russian, 4 British and 3 American. Not to difficult and really exciting your never just wondering "what do i do now" you are always do something. The campaign seems some what un-interested through the Russian point of view but as you progress to the British and American you get a strong feeling that this is a large scale war.
Now the part i love the most, the multilayer. It consists of 5 different types of games. The usual Team Deathmatch, Death match, Capture the Flag. In addition it has Headquarters, which consists of a team getting to a box (headquarters) which is randomly put on the map and you need to hold it for around 120 seconds in-order get 120 points, and usually this is up to 400. The last is 'Search and Destroy' where a team will plant explosives and the other has to defuse on either A or B, this is really just counter-strike type style.
Lastly a few minor things, the graphics are great maybe not as good as Gears, Cod3 or R6 but are still really good considering that this was a lauch title. If your thinking that it may be hard to get into an online game due to the time it was released, then your wrong because it is still quite easy to get onto a game and their are still a handle of competitive team based leagues for CoD2 now.