Submodules (discussed in the Pro Git Book), helps manage repositories nested within a main repository:
Submodules allow foreign repositories
to be embedded within a dedicated
subdirectory of the source tree,
always pointed at a particular commit.They are not to be confused with
remotes, which are meant mainly for
branches of the same project;
submodules are meant for different
projects you would like to make part
of your source tree, while the history
of the two projects still stays
completely independent and you cannot
modify the contents of the submodule
from within the main project.Submodules maintain their own
identity; the submodule support just
stores the submodule repository
location and commit ID, so other
developers who clone the superproject
can easily clone all the submodules at
the same revision.