These servers vary in game modes, ranging from the standard rotations including capture the flag or payload, to game modes you won’t find on official servers, like Death runs and Jump maps. These servers are hosted by other members of the Team Fortress 2 community, hence the name, rather than Valve directly.
Rounding out a nice ‘3 C’s of TF2’, there are also community servers. Since the Meet Your Match update, most servers are split between either casual or competitive, however there is another.