It's a Massive Multiplayer Online sort of game, but instead of fighting monsters or wandering the streets, you play puzzles with other people, surrounded by a piratey theme. It's a lot of fun, and the puzzles are quite enjoyable either as small diversions or as something to dig into and really excel. The community is great, and the overall feel of the game is very welcoming.
As always, there are jerks online, but YPP tends to be better than most games I've played so far as that goes. Also, since it's an online game, you need a good internet connection and decent computer to get the most out of the game. It's much kinder on the requirements than something like World of Warcraft, though.
I'm not much of a fan of MMOs, but this one is just fun.
If you get a minute, you ought to check it out. If nothing else, it's free to play, and it can be a nicely structured alternative to Solitaire or Yahoo games.
Below is a link to their home page. The "Play Now" button will get you up and running, but there are a ton of other good links there. I especially recommend the YPPEDIA link, as it's full of good information about the game. Sometimes it's best to jump in and play, sometimes some scouting is good. From this page, you can do both.