VACUUM FREEZE vs. VACUUM FULL
Here's a short concise answer.

Vacuum full takes out an exclusive lock and rebuilds the table so that it has no empty blocks (we'll pretend fill factor is 100% for now).

Vacuum freeze marks a table's contents with a very special transaction timestamp that tells postgres that it does not need to be vacuumed, ever. Next update this frozen id will disappear.

For instance, the template0 database is frozen because it never changes (by default you cannot connect to it.) 

Every so often the autovacuum daemon will check a database and its tables to see what needs to be vacuumed. If a table is vacuum freeze'd and then never updated, the autovacuum daemon will simply pass it by. Also the "wrap around" protection in postgresql will never kick in on that table either.

tl;dr freezing marks a table as not needing any autovac maintenance. The next update will unfreeze it.