AutoPairs modifies the behavior of certain keystrokes, to help you keep paired characters such as parentheses properly matched. For instance, when you type a left parenthesis, AutoPairs will type the right parenthesis and a left arrow for you, so that you are ready to type what goes between the parentheses. This and other pair macros can be turned on and off individually, and configured differently for specific applications.
AutoPairs is a system preference pane, as well as a background application. It will not be visible in your Dock.
Version 3.0 is a Universal Binary, and has been rewritten with newer techniques which should be compatible with Leopard. However, it is not compatible with Panther or earlier.