This question comes up often, so I'll post it here for the record:
I started this site in 2003 for people, specifically Westerners, who have zero experience with Buddhism. I'm talking utter ignorance like my dear, sweet departed grandmother who thought Buddhists were orange-robed Christians—her worldview literally couldn't comprehend a non-Christian existence.
Many of us are raised in cultures that completely ignore Buddhism; at most it's a footnote in the local bookstore, a "religion for Asians", or a special on evening television. In Western pop culture, it's a religion for weirdo celebrities, and most middle Americans would be hard pressed to tell you the difference between Buddhism, Hare Krishna, Hinduism, and Kung Fu movies.
This site was founded on the idea that Westerners who come from these types of backgrounds need a safe place where there are no stupid questions. Do Buddhist celebrate Christmas? Was Buddha a real person? Is the fat Buddha in Chinese restaurants a God that Chinese people worship? How can I be happy?
Anything else that this site has become is a direct result of the people who choose this as their online home. We have monks, laypeople, born again Christians, Muslims, and people from every internet-connected nation on Earth visiting here. It's going to be wild and diverse and crazy; that's what we love about it.
There is no single answer to the question "What is this site?". It is a place where the moderator team and myself will keep things nice so that everyone can talk about Buddhism at any level, freely, and without fear of being ripped a new asshole for saying something someone else disagrees with or thinks is stupid. Nothing more, nothing less.