Rear set essentially is a term for a kind of footrest that was used in racing because they were positioned further back than the original footrests, ie; they were set to the rear. They were generally higher too but a rear upset wouldn't have sounded as good and might have been taken the wrong way
For some reason a lot of people these days seem to call the std footrests rear sets but I've no idea why as the entire definition of the term means non standard, somewhere along the line some muppet must have got the term confused and it spread out among the population.
In the old days the rearset you bought would have one position but these days they're adjustable to many different positions to suit the riders preference, the main reason is to improve ground clearance and body position for racing and also important these days is that all the parts are available seperately so that you can replace components easily in case of a crash which obviously is a lot more prevalent on track than on the road.
