UK Visitor Visas are required for some foreigners who wish to enter the UK for short stays. Whether you need a Visitor Visa depends on your nationality, the purpose of your visit and how long you need to stay. The UK Visitor Visa category includes a number of specific visitor visas that allow people to holiday, visit family, get married, study a short course, and sit the PLAB as well as many other purposes.
Generally Visitor Visas are valid for 6 months. In some cases, they can be issued for longer periods such as 1, 2, 5 or 10 years. However, those with longer term visas can still only stay in the UK for periods of six months at a time, other than Academic Visitors and parents of children at school.