Weird game, not necessarily bad.
You start out as a worm-like creature called "Boy". Boy can stretch his body as far as you want it to go. The farther you stretch, the more you contribute to the size of another worm-like creature called "Girl". Girl is pretty much a gigantic Boy. Girl is always trying to get to another planet, but she needs Boy's help to get there. Even though this might sound like an impossible task, you have help. A lot of help. Every person who owns Noby Noby Boy is also trying to stretch out Girl. So you're pretty much working with people all around the world to reach new planets and play on new maps. At the moment Girl has reached Jupiter and is now heading for Saturn. I love this concept of Noby Noby Boy because the feeling of everyone working together is really satisfying.
Each map you play on is different. Different items you can slither through, different creatures that you can take for a ride, eat or even combine. Noby Noby Boy doesn't REALLY have a purpose, it's pretty much just a relaxed game which I like.
Noby Noby Boy does offer offline multiplayer. It doesn't really differ from your single player experience but it does allow you to stretch, eat, explore, etc. with your friends.
All in all, Noby Noby Boy is the right choice if you have some money to spare and are looking for a game that's unique and different in it's own special way.