LOOP | HASH | MERGE are Join hints, specifying that the join in the query should use looping, hashing, or merging. Using LOOP |HASH | MERGE JOIN enforces a particular join between two tables. LOOP cannot be specified together with RIGHT or FULL as a join type.
You should always use INNER JOIN. Let the query optimizer decide whether it wants to do a LOOP, MERGE, or HASH join. In almost all cases the optimizer will make a better judgement. Which one will be used and when can be found from my presentation http://sqlbits.com/Sessions/Event4/Understanding_Graphical_Execution_Plans.
(Edit: The link to the video presentation on that page is broken, but Archive.org has archived it here)