Essentially the same thing as 120 film except twice as long. Many Medium Format cameras that use 120 will also accept 220 film, but not all - because the film is both longer and heavier it increases the tension in the winding mechanism of the camera and it requires the spooling to be a little beefier.
Also, 220 film is not paper backed from start to finish but rather backed at the start and finish, but not in the middle. This is to compensate for the longer film in order to make the roll not too much larger than a 120 roll - the importance of this is that certain cameras (early Hasselblad film holders, for example) are not 100% lightproof from the back - expecting the paper to protect the film. Put 220 film in one of these and you're going to get a surprise.