Simple Question #004a: What is a Church?

Is it the building? Is it the leader? Is it the people? Is it the community efforts? What really is a Church?

I know that Christ said, For where two or three are gathered together in My name, I am there in the midst of them. So what about when I'm all alone? Why isn't that a Church too. What if I'm standing alone at the altar of a well-known cathedral and praying earnestly? I guess that's not really a Church... but why?