Difference between rpm -i and rpm -U ?

[Dan Thurman]

Folks,

What is the difference between rpm -i and rpm -U ?

If I have a bunch of rpm files and use -i, will it remove
previous versions of packages or will it retain older packages
and install the newer package on top?

I assume -U looks for older packages, removes it if found,
and then installs the newer package?

I just want to make sure that I do no end up with filesystem
bloat choosing the wrong arguments.

Thanks!
Dan