Hi,
Adding some comment to what flatulent said.
"The hot swappable part of the system can be removed and fitted without disrupting the system". Thing for example in a USB mouse. You can connect/desconnect it when your PC is running and it will work, you don't have to restart your PC like in the old days when you have to fit your mouse in your Laptop and things like that.
I was involved in the design of a cPCI card. These cards were fitted in a chassis (with its backplane, PSU, Fans...), they were hot swappable, most of the specs were in the cPCI Specifications, signals you need (PWR_GOOD#, HEALTHY#...), length of pins in connectors, protocol in extraction/removal.
Regards,
-Maestor