nfs export table on many UNIX-like systems. Here's all you need to know about the basic format in a nutshell:

/path	allowedhost(access)

$ man 5 exports for details.

tftv256's description is correct for Linux (if you wish, you can replace "allowedhost" by "@allowednetgroup"), but the format for HP-UX is very different:

/path -access=allowedhost1:allowedhost2,otheroptions

Two useful other options are "ro" (export the directory read-only) and "root=host1:host2:..." which grants root on certain other machines full access to the exported directory, rather than treating root as an anonymous user.