There are 2 types of AP, i.e. autonomous and lightweight. You only need a WLAN controller if you plan to use lightweight APs. As the name suggests, the controller manages all APs associated with it in a configuration known as split MAC where the AP handles real-time events while the controller takes care of security and other "back-end" processes. In contrast autonomous APs have full IOS and each handles all processing. This is just a teaser but there's plenty of docs on Cisco! Theo
