Plus is a specialist British market for small cap shares. Its closest competitor is AIM. It was formerly known as Ofex.
Plus suffers something of a disadvantage in competing with AIM in that it is not part of a large exchange. This means both a lower profile and fewer brokers able to deal in Plus quoted securities.
Plus is a fairly cheap exchange to list on for a company that does not want to raise capital as a prospectus is only needed when issuing new shares.
There is more information on the Plus Markets Group website.