While it has a number of features, the one I mainly use is a tactical solving training element, and I prefer to not work against the clock, although I will sometimes test my vision in timed tests.
You get a solving rating, and the site rates puzzles and then tries to match each solver up with a puzzle of roughly an equal level. If you correctly solve the puzzle your rating goes up and you'll start to get harder puzzles, while if you get it wrong your rating goes down and you'll get easier puzzles. (the puzzle rating also goes up or down depending on whether it is solved)
Solving puzzles like this helps a player to develop their imagination and improve their calculation skills. Or put another way, you work on diverse ideas, and how to accurately employ them.
Here's a puzzle that I was given today. See if you can solve it, I'll post the answer tomorrow. And improve your tactical ability by joining Chess Tempo and solving puzzles.
It is white to move, black has just played Rg8-g6. You don't know what your objective is (mate, winning material, saving a lost game etc), you just have to find the best line for white.
TIP: make sure you look at your opponent's ideas and resources.