Whether you need a visitor visa depends on your nationality, the purpose of your visit and how long you want to stay in the UK. When applying you will be informed if you do not need a visa.
The standard UK Visitor Visa category has replaced a number of specific visitor visas categories that allowed people to holiday, visit family, get married, study a short course, sit the PLAB test as well as many other purposes.
Generally, Visitor Visas are valid for 6 months. Some categories grant you a visa of 30 days, 11 or 12 months and you can also apply for multiple entry visas for 2, 5 or 10 years.
But, even with a multiple entry visa, you should generally not stay in the UK for longer than 6 months at a time other than Academic Visitors and parents of children at school.
If you regularly visit the United Kingdom, you can apply for a multiple-entry visa as a visitor that is valid for 2, 5 or 10 years.
TIP: Do not apply for a multiple entry visa on your first application.