An automounter automatically mounts/unmounts file systems based on its configuration table; the table maps local mount points to specifications of remote mounts. A simple example:
/home ourserver:/export/home
Automounters may allow all kinds of interesting gadgetry within these tables: for example,
autofs allows several forms of wildcards, it allows tables to include other tables, and it allows tables to be taken from flat files or NIS/NIS+, in any combination.
Automounters aren't actually limited to NFS mounts; at least the Solaris autofs can work with other types of file systems.