The UK fiancé / fiancée visa is for those couples not yet married, but who want to marry and live together in the UK. It can only be applied for from abroad. It cannot be applied for from inside the UK. Exceptionally during peak Covid, it was possible to apply from inside the UK, but this is no longer the case.
The fiancé / fiancée visa is a 6-month visa, and the requirements are the same as those for the spouse visa, except for one critical difference: You must show that you intend to marry in the UK within the 6-month validity of the visa.
The foreign partner is not permitted to work on a fiancée / fiancé visa, and once married, they will need to apply for a spouse visa extension from inside the UK. This application is referred to as FLR(M). Once the foreign partner has obtained the spouse visa, then work is permitted.
Beginning with the fiancé / fiancée visa, the sequences of applications is as follows:
1. Fiancé / fiancée visa
2. Initial spouse visa (FLR-M)
3. Spouse visa extension (FLR-M)
4. Indefinite Leave to remain (ILR)
5. British citizenship
So, the fiancé / fiancée visa is essentially an additional step, added on at the beginning of the spouse visa route. For that reason, it is a more costly route. If you are able to marry outside the UK, and apply straight away for the spouse visa, this would be a cheaper option in terms of visa fees. Although you should of course consider the overall costs of conducting a wedding abroad (i.e. flights, hotels, etc.) and you may find a fiancé / fiancée visa is still cost effective.
You can find a more detailed explanation of the differences and considerations when choosing between the spouse, fiancé, unmarried partner, and marriage-visit visas, here: UK Family visas - What's the difference between them? (owlandsnake.co.uk) And also, here: UK Family visas: Spouse vs. Fiancé visas. What's the difference? (owlandsnake.co.uk)
What are the fiancé / fiancée visa requirements?
The financial requirement is the most complex part of the fiancée / fiancé visa application, and also the most important. It can be met in various different ways, so it's difficult to summarize in a just a few bullet points. But by way of an overview: