A bladder tank is a storage device and a pump at the same time. What it does is store water by filling a balloon which are bladder inside a steel or plastic tank. As the balloon fills the air trapped in-between the balloon and the wall of the tank stores energy as the air is compressed until it equals the incoming water pressure. When you open a faucet or flush a toilet the line pressure drops and the trapped air forces water out of the balloon at a high flow rate. You want to make sure you have a check valve between the bladder and the water supply or you will end out adding pressure to all of your neighbors houses. Bladder tanks are great for fixing pressure fluctuations especially when someone flushes a toilet when you are in the shower.
Using a bladder to solve low water pressure problems
Capacity Charts are available here
Regular storage tanks can be found here