How do I sync two folders in OS X Lion? I have some files and folders on a USB drive connected to my iMac...I would like to sync them with a folder on my iMac...how do I do that?

Is there built-in functionality in Lion that allows me to do that, or do I have to use a 3rd party solution?
arRsync is a free OS X GUI to the rsync command line utility. It will get you started on the path to rsync enlightenment. You can save common sync operations to recall them at a later time for easy rsync. And it supports the common rsync use case scenarios: push A to B but leave stuff that's in B but not A in place, push A to B and delete stuff that's in B but not in A, make A and B look the same (with a conflict report).