Retrieve post tag by tag ID or tag object.
If you pass the $tag parameter an object, which is assumed to be the tag row object retrieved the database. It will cache the tag data.
If you pass $tag an integer of the tag ID, then that tag will be retrieved from the database, if it isn't already cached, and pass it back.
If you look at get_term(), then both types will be passed through several filters and finally sanitized based on the $filter parameter value.