What is the difference between "light-weighted keys" and "hammer-action keys" on keyboards? They both have the feel of a piano,right? I am looking for a piano keyboard and not really sure what to buy (I am not a beginner though). Thanks in advance.

Keys on an acoustic piano are heavier because when you hit the key it causes a whole series of actions, resulting in the hammer striking the stings. Some keyboards can come pretty close to this feeling, but none of this is necessary on a keyboard, so most have "light-weighted" keys.
If you intend to seriously study piano, nothing can replace the feel and sound of a true acoustic; however, if you have to get a keyboard, get one with the hammer-action.

I just studied up on this through the internet and talking to some stores that sell instruments(our church may be buying a digital piano). Yamaha makes a great piano. I'd look at the YDP223. Great piano!