If you plan to travel to the United Kingdom for a short length of time usually no longer than 6 months and for a temporary purpose, you will need to apply for a Visit Visa.
Applications can be done at a British Embassy/High Commission or visa application centre in any country, not necessarily the one where you normally live.
The most popular visit visa is the Standard Visitor Visa, which can be used for several reasons. However, there are also other types of visa that may be requested for specific purposes: the Marriage Visitor Visa, the Permitted Paid Engagements Visa and the Parent of a Tier 4 Child Visitor Visa.
Each category has its own eligibility requirements, its specific hard copy application form or type of online application and supporting documents.
You are not allowed to switch into any other category of visa other than that for which your visa was given.
You can start the visa application process up to 3 months in advance of your planned travel date. The UK Border agency advises you to start the visa application process at least 5-10 weeks prior to your planned travel date.
To learn how to apply for all UK visas, click here