This visa is available for those whose family members and partners stay in the UK for 6 months or even permanently. Depending on what country you are from, and you the rules of application vary.
For non- EEA Zone nationals, only certain relatives can apply to join them or remain in the UK. Fiancés, spouses, civil and unmarried partners, for those under 18, parents, grandparents, adult children and guardians are eligible to apply for his type of visa. Your family can also apply to join you if you have UK citizenship or an indefinite leave to reside in the UK. They can stay for six months or more and will need to prove their relationship with you. They must have a good knowledge of English and have a clean criminal record.
Note that their stay in the UK will largely be determined by the type of visa you are staying in the UK on and upon your exit from the UK, or divorce or demise, they also have to leave the UK or reapply for a new UK visa.
In addition to standard travel documents, they will need to provide evidence of relationship to the family member they are joining, and proof that they can support and maintain themselves financially and otherwise. They will also need to prove that they have a sufficient knowledge of the English Language.