Sort merge joins and hash joins store data (rows) in the PGA, nested loops use the buffer cache in the SGA. Thus, merge joins and hash joins don't have the overhead of latching and reading blocks into the buffer cache.
A sort-merge join can be more effective than a hash join if the hashed data of a hash join does not fit into memory.
Cartesian joins are executed in one of these three join methods.
See also
When using an adaptive plan that involves a hash join or a nested loop, the statement executor might choose change from one of these methods to the other.