Basically an info type of site to find out about local businesses where the info is mainly submitted by users, such as reviews. Extremely useful for people who want to try new places or visiting a new area. I ended up using zagat mainly, mostly cuz it's integrated into google maps now.
I use it for user reviews of restaurants. I mostly look at the negative reviews so I know what places to avoid.
It's great when traveling somewhere new. I used it in Chicago to find every good hot beef sandwich place. I was sick of hot beef sandwiches when I left. So far I've reviewed 6 restaurants.