They are appointed by letters patent to be one of "Her Majesty's Counsel learned in the law". They are not a separate type of lawyer. They are more than long serving lawyers, because their status is given by the Crown and recognised by the courts.
Queen's Counsel have the privilege of sitting within the Bar of court, and wear silk gowns of a special design (hence the informal title Silks). The special robes are the reason why becoming a QC is often called "taking silk".
A QC's status means they generally charge higher fees than ordinary barristers, and always have another barrister as an assistant. This assistant is called a "junior" even if the junior has been a lawyer longer than the "silk".