Retrieves category data given a category ID or category object.
If you pass the $category parameter an object, which is assumed to be the category row object retrieved the database. It will cache the category data.
If you pass $category an integer of the category ID, then that category 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.
The category will converted to maintain backward compatibility.