I've heard it's a good place to start for games programming but even after reading the website's first few tutorials I still don't really get what it is.
Is it just a library of extra commands or can you use it to design graphics and GUI?
SFML is a multimedia library it acts as a wrapper for several low-level libraries (and os) and integrates them into several modules i.e graphics , audio , network , window.
*Its not just a graphics library.*
Now , Please don't ask later which you should choose between sfml or sdl or etc.
There is such a question asked here every two weeks or so, either try each one or dig up this forum for such x vs. y vs. etc.