Kellie,
A leisure battery allows you to operate 12 volt appliances in your caravan when away from electrical hook-up - things like water pumps and low voltage lights. And if you have one - a caravan motor mover.
In addition to this:
If you have a UK caravan - then it is very likely that a battery is required to supplement any moments of high current demand on the 12 volt system - and the battery will also smooth the output from the caravan power pack. If your need for a battery is just to provide a supplement to the power pack, then pretty much any 12 volt battery will do (as many have testified).
If you have a European caravan - then most of them are designed to run their appliances without the need for a battery to boost the current. But if you want to go wild camping, or fit a motor mover, then you'll need a leisure battery.
Robert