What is the difference between FHD and UHD?

HD is 1080i. FHD is 1080p. UHD is 4k (UHD is the consumer standard, 4k is the commercial standard - it's basically the same thing, they're just different ways of defining it).

You're not going to notice the difference between them unless the display iis larger than about 60 inches - certainly not on any phone or tablet. But the higher the definition, the larger the file will be, and the longer it will take the phone to process the picture unless you're shooting in raw format (which is fast and can be converted to anything else later).

What's the best setting to use? If you're going to be showing or looking at the pictures on a 24 or smaller screen, HD is more than enough. If you have an 80 4k TV and want to show the pictures on that, use UHD.