Xsm is a session manager. A session is a group of applications, each of which
has a particular state. xsm allows you to create arbitrary sessions - for
example, you might have a "light" session, a "development" session, or an
"xterminal" session. Each session can have its own set of applications. Within
a session, you can perform a "checkpoint" to save application state, or a
"shutdown" to save state and exit the session. When you log back in to the
system, you can load a specific session, and you can delete sessions you no
longer want to keep.

Some session managers simply allow you to manually specify a list of
applications to be started in a session. xsm is more powerful because it lets
you run applications and have them automatically become part of the session. On
a simple level, xsm is useful because it gives you this ability to easily
define which applications are in a session. The true power of xsm, however, can
be taken advantage of when more and more applications learn to save and restore
their state.