Different classes have different abilities, it is best to have a group of four with one of each class, getting a group of four to co-operate is extremely difficult if not impossible unless you are good friends with them.
Miner
Can Carry: 200 Ore, 8 Weight. (The class menu says 10 but it's really 8)
Can Build: Team Solid, Team Force Field and Ladder
Special: Fast dig speed. 'Digs like a freakin' terrier!'
Prospector
Can Carry: 200 Ore, 4 Weight. (The class menu says 5 but it's really 4)
Can Build: Team Solid, Team Force Field, Ladder and Beacons
Special: Prospectron. (Detects Gold and Diamond's)
Engineer
Can Carry: 350 Ore 4 Weight. (The class menu says 5 but it's really 4)
Can Build: Team Solid, both team Force Fields, Road, Ladder, Jump, Shock, Bank and Beacons. (Basicly every block but the other team solid)
Special: Deconstructor. (Deletes every type of block placed by your team except TNT, It does not give you the ore back)
Sapper
Can Carry: 200 Ore 4 Weight. (The class menu says 5 but it's really 4, also note that he can place a max of two TNT at one time)
Can Build: Team Solid, Team Force Field, Ladder and TNT
Special: TNT and Detonator. (When a TNT is activated it will destroy all blocks and players besides Gold and Diamond in a 5x5 area around the block, Note: It will destroy lava but unless you remove all of it more will pour in from the sides)