The opposite of a fat driver!
But seriously... Oracle provides two versions of its driver: thin and OCI (thick). Thin is based purely in
java and is generally very stable whereas OCI is written in more Oracle native code and is considered more powerful.
I've found most people use thin drivers although there are some excellent reasons for using OCI. Here's a short post listing some reasons:
http://forum.java.sun.com/thread.jspa?threadID=629750&messageID=3622539