First of all, UK marriage visas are necessary for individuals who want to become residents in the United Kingdom based on their marriage to a British citizen or individual that legally resides in the United Kingdom. If the person meets specific criteria, they will get a permit to reside in the United Kingdom.
Individuals from countries outside the EEA and EU must apply for a settlement visa to the British Embassy, Consulate or High Commission in the place where they are living. In case everything is fine, the applicant will receive a multiple entry visas that lasts for two and a half years. In case you are in the United Kingdom and you have a visa which lasts for over 6 months, then you can also apply for a marriage visa from within the United Kingdom.