What is rapper, anyway?
A rapper is a flexible strip of sprung steel, usually around two feet long, with a handle at each end. Though often called "swords", they aren't really - popular rumour maintains that they were used by the miners to scrape the mud off the pit ponies when they came up from a shift in the mine.
And you dance with these things?
Yup. Five people hold a sword handle in each hand, to form a circle. Various "figures" are performed, in which the swords are twisted and woven into patterns. These vary from the relatively simple, to sufficiently complicated that it involves dancers jumping, or even somersaulting, over the swords to untangle themselves again.
The music used is mostly traditional Northumbrian tunes. They're fast and lively, and for some reason people always seem to think they're Irish.
Ideally, rapper should be performed on a wooden floor, so the dancers' fancy stepping can be heard. If you ever hear anyone describing a pub, and they list "nice floor" among its attributes, chances are they're a rapper dancer.
Throughout the dance, the dancers never let go of the swords. This is very important. Trust me - let go, and you could end up unconscious on a pub floor in Burton-on-Trent. I speak from experience.
As far as I can tell, these dances evolved because 5 drunken miners in a North East pit village discovered that, on account of living in the 19th Century, they had a bunch of rappers and nothing better to do. A slightly more detailed history, however, can be found here.