The Interstim device is a neurostimulator. It has a control unit that's implanted in the upper buttock area on one side or the other (mine's on the left), and a lead with four electrodes that goes against the nerve bundle from the bladder and bowel right where it enters the spine.
Once it's implanted, the control unit sends electrical impulses to the electrodes on the lead, which causes gentle stimulation of the nerve. When it's adjusted correctly, I rarely even feel it, though sometimes when I move I'll have a sensation like someone is tapping on my perineum.
For reasons that aren't completely clear to me (and I don't honestly know whether they're clear to anyone), this sort of nerve stimulation helps with urinary incontinence, urinary retention, and bowel control problems that are caused by neurological problems. The way the urologist that did the implant explained it to me is that it essentially gets the nerve used to a higher level of signals, so the oversensitivity that causes urgency is reduced.
Mine has helped a lot, and with the recent adjustments to the programs seems to be back to working as well as ever.