There's a professor at my school whom everyone says is "famous." I hadn't heard of him before I started my master's program, but now that I've started, I see his name come up over and over in graph theory. I went to the library to check out a book on graph coloring, and he had written the first chapter. I looked up a result that might help me on a hard homework problem, and I found a paper that he had written. A student visiting my university said that the other university he was looking at had proudly encouraged him to come to a talk at their school, given by none other than this professor from my school. I think famous means that once you find out who they are, you keep noticing their name over and over again. I'd recommend talking to faculty at your school to get a better idea of who is "famous."