In order to use WiFi Password Recover you need to download its complete dictionary of WiFi passwords, which is around 60 megabytes. This provides the app with all the information it needs.
The compatibility list in WiFi Password Recover is pretty extensive, and includes routers from several brands and different protocols. That said, this doesn't mean it's infallible. This app is able to give you the password for most WiFi networks, but not all of them.
WiFi Password Recover is a great app, similar to Router Keygen. It lets you do almost the same thing: get the password for a WiFi network in a pretty easy way.
Requires Android 2.1 or higher