I have both a full tens unit and a little portable modified one that I found on eBay. I really love the small one. I don't have it with me at the moment so I can't tell you what kind it is or anything. I'll look for it and post the model later.
To use laymen's language, a tens unit sends mild electrical impulses to certain parts of the body to block pain signals. They are mostly used by physical therapists as part of therapy.