Retrieves a 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 from 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 passed back.

If you look at get_term(), both types will be passed through several filters
and finally sanitized based on the $filter parameter value.

function get_tag( $tag, $output = OBJECT, $filter = 'raw' ) {
	return get_term( $tag, 'post_tag', $output, $filter );
}