They work by creating a vacuum which increases the blood flow and helps an erection. They ARE widely used in the medical profession, but in cases where a man just needs a little help to get going there is nothing wrong in using the over-the-counter (or rather over-the-net) models which are widely available without resorting to visiting clinics etc.
Speaking for myself, I bless the day when I made a small investment of �18 and purchased mine from a mail-order outfit -
http://www.formint.co.uk . They still have them, called Lust Buster listed under `boys toys` as well as others which are only a little more expensive.