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. Tankers can also drop off their water in a dump tank, which is like an above-ground swimming pool. The pumper then sucks the water out of the dump tank through a hose. Rescue trucks sometimes look like ambulances, and sometimes they're much bigger.