Okay, I've read a small article on wiki which explained AJP a bit. Basically this protocol is used for communication between Apache and Application server. But why may one need it in the first place? Why not use just plain http?
Answer
More specifically, and not really covered in that ehow article, there's some non-trivial overhead to parsing http headers on requests and creating them on responses. If you are fronting your app server with a web proxy server, AJP allows you to skip that extra parsing and just pass efficient binary representations of the headers between the proxy server and the app server. There are other benefits with things like simple connection pools and tight integration if you use apache/mod_jk to tomcat.
Client <- http/s-> Proxy <- http/s -> App
vs
Client <- http/s-> Proxy <- AJP -> App