The "loop" is a stroke that generates a lot of topspin (with either the forehand or backhand). There are slower, spinnier loops as well as faster but not-as-spinny loops. The loop style is the most popular playing style in table tennis with a strategy heavily reliant on the loop stroke.
The loop is a great stroke because the heavy topspin helps the ball arc downward back on the table, yet it can still go very fast. The combination of speed and spin make it a shot that is both deadly (due to the speed), and controllable since it arcs back down to the table. In contrast, the smash has little topspin so there are fewer safe opportunities to use the smash effectively.
You can also add sidespin to make the loop "hook" to the side.
Watch some Loop Training Videos
See the Loop Stroke
section in the Coach Wiki
Coach Wiki Pages