Usually each truck has a different job. The trucks carry all of the tools the firefighters might need, and often they carry some water in a tank also. Pumpers carry some water, a lot of hoses, air packs, and usually a ladder or two. Inside the truck is a large, powerful pump to push water through the firefighter's hoses. The truck engineer runs the pump.
Ladder trucks carry ladders.
These days, the ladder truck usually has a very big ladder on the back and some smaller ladders. Usually a pumper has to pump water to the ladder truck. Tankers carry water - and lots of it. Tankers are useful when there's a fire someplace where there's not a fire hydrant nearby. Usually the tanker sends water to the pumper through a big hose.