Some GTA 4 multiplayer details from the official site.

Team Deathmatch (2-8 teams)
Players work as a team to rub out other teams for money - and the crew with the most cash at the end of the round wins.

Team Mafiya Work (2-8 teams)
You’re a member of a crew doing contract work for the mafiya - including such plum gigs as escorting wanted men, picking up MacGuffins, and stealing cars. Work as a team to complete contracts before the time period ends and get the rewards.

Team Car Jack City (2-8 teams)
Cars spawn parked around the map and the teams must steal them and take them to a drop off point. The cash teams get for dropping off stolen vehicles depends on their condition - teams get less cash the more damaged they are. The exception to this are special bonus cars, stuffed with drugs, which give a set amount of cash on delivery no matter how damaged they are.

Cops n’ Crooks (2 teams)
A team-based mode in which the crooks must get their boss to safety, while the cops have to take him down. This mode has two sub-modes: All for One and One for All.

Turf War (2 teams)
A timed capture-the-base mode. There are a number of bases around the map that you have to take by standing on them for a short period of time. The more players that are on a base, the faster it can be captured. The more bases your team owns, the faster your team’s cash score increases.