What is the difference between UProperties and FProperties in the Engine?

Anyone able to give a deeper engine understanding of the difference between UProperties and FProperties?

1 - I found an old post that said FProperties are newer, but I have no idea if that is actually true because the post seemed old.

2 - I thought FProperty meant it was a struct, but it is not, both are classes.

Is it something about the reflection system? Where properties are converted from UProperties to FProperties on the default object or something?

Thank You