PT3 does quite a lot so I'll just try to give you a general overview - for specifics on features, we have quite a bit of
documentation and several
tutorial.
PT3 keeps track of everything that happened in every hand you've ever imported and so PT3 has two main ways to be used:
(1) For post-game analysis
(2) For in-game assistance.
In the first case, when you're not playing, you can review things like your overall statistics, overall statistics of opponents, look at graphs and reports of any number of players, replay specific hands, and so on. The goal here is to get better away from the table and improve your game while you're not playing.
In the second case, when you are playing, PT3 will display on the table any statistics you may choose to see. You can then use these statistics (which are only based on hands you've imported into your database) to infer things about your opponents' playing tendencies. (Quick example: If you've seen a guy play 1000 hands and he has raised 3 times in those 1000 hands, if you open and he re-raises you and you're not all that deep, it's a pretty easy fold unless you have exactly aces.)
Anything that PT3 does for you, PT3 can also do for your opponents, but remember they would need to have imported hands into their own database that you have played in order to get any data on you personally.