http://www.conitec.net/beta/ain-handle.htm gives a detailed explanation about handles. Here you can even read how to determine an engine objects type by it's handle.

http://www.conitec.net/beta/ain-ptr_for_handle.htm tells you about ptr_for_handle() and gives a short example.

Roughly spoken you use handles allmost always by saving a handle in an entities skill by "(my->skill)[40] = handle(you);" and retrieve it later by "you = ptr_for_handle((my->skill)[40]);" where "you" can be replaced by any other pointer to an engine object and 40 is to be replaced by any value between 0 and 99. Examples are in Lite-C Syntax.