Add Additional Submission Processing To "Create Node" forms
In Drupal 6, this was done in hook_nodeapi.   

In Drupal 7, this is done in hook_node_update &amp; hook_node_insert

hook_node_update($node)
This hook is invoked from node_save() after the node is updated in the node table in the database, after the type-specific hook_update() is invoked, and after field_attach_update() is called.

hook_node_insert($node)
This hook is invoked from node_save() after the node is inserted into the node table in the database, after the type-specific hook_insert() is invoked, and after field_attach_insert() is called.

there is also hook_node_presave($node) if you want to do something before its been save

More information on api.drupal.org &amp; link to the D6 to D7 upgrade changes