It's great for aggregating ideas, projects, products, etc. onto customizable "pinboards," similar to offline bulletin boards. But that's just the practical aspect. Pinterest is a lot of fun, because you get to see people's pins from all over the world and discover all kinds of new things.
It's hard to describe; you have to try it to see just how fun and addictive it can be.
If you'd like to see a sample collection of pins, go to my HubPages profile for a link to my Pinterest page.