Hi,
For a site:
An outbound link is the link which starts at your site and ends to any other site.
An inbound link, starts from any other site and ends at your site.

For a web page:
An outbound link is the link which starts at your web page and ends to other web page (may be other page of your site).
An inbound link, starts from any other web page and ends at a particular page of your site.