A session (HttpSession) is a kind of collection object whose life cycle is managed by the
servlet container. It consists of a collection (think Map) that holds Object references indexed by
String keys, plus a bunch of bookkeeping variables that let it track the time of last access, send events to listeners, etc..
If the objects you add to a session are serializable, the servlet container will be able to write them out to disk to save memory.
If you want to dig into the detains, look at the source code for
Tomcat.
Bill