I will thus try to answer it in a nutshell:
Revolut is a bank, a normal bank. But unlike traditional banks it is one that operates purely on a digital basis through an app.
The company begun in 2015 as a FinTech startup offering a prepaid debit card. It was then a service whith which you basically could manage your prepaid debit card online and a few extras.
After the initial success they added a bunch of other services like insurance and a few others and as the amount of customers increased they applied for a banking licence, which they got.
Revolut can offer cheaper services in many areas because it lacks the physical infrastructure of traditional banks and the cost of customer acquisition is very low.
It is considered an example of what’s called unbundling and re-bundling of finance:
First it was a startup that unbundled the service of prepaid debit cards from traditional banks
After it’s growth it transformed itself again into a normal bank, and that’s what’s called “re-bundling” as it does now bundle the services of a traditional bank.
So, that’s what it is: A digital-only bank.