I think a combination of why is this company good and how you will be excellent for the company(Insert Monty Burns voice if it helps). This can be a number of different things though I also think part of the interview process to get to see how things are done at a company.
The "tell me about yourself" for me tends to be a little about where I was born and grew up, quickly noting university before going through various highlights of past work experience, what did I enjoy, what did I do because I was asked to do it and I thought it was stupid to do it this way but I did it that way anyway, and how I got to being before you today.
I tend to focus on what skills did I get from each position, what did I like there that is worth passing on, what did I not like that I hope isn't passed on here, what am I looking for in terms of a work environment outside of salary.