Roller skating is travelling on smooth terrain with roller skates. Inline skates and skeelers are special boots with four, respectively five small wheels in a row after each other. An older, more primitive kind of roller skates have two axles with two wheels each, per skate, without boots; they are tied to regular footwear.
Thus it is similar to ice skating, with the metal blades replaced by wheels. It is often done for recreation and as a sport, but, more often than ice skating, it can be done functionally for moving around (it is faster than walking).
It is done on the same places as skateboarding: on the road or sidewalk (not always allowed), and on special tracks and areas, including skate parks and half-pipes.