I'm guessing this is what hold the rear wheel to the bike, and part of the frame?
I don't know the term either, but if it's that you can't replace it and you can, but it's not worth it.
You can find a galore of used parts on Ebay, none on CL
I also recommend that you swing by a local bike shop, after you explore the bike shops and talk to them. For a beginner a LBS can be a lifesaver, but make sure you shop around for one that's cheap, carries cheap and high end parts, and will do work or service for a good price. I go to one that will touch up my bike for free because the people know me well, and the parts or items I buy there are $1 over online prices.
a 1998 Trek will be able to take most if not all parts, give or take an adapter, with no difficulty. Research and read up on certain terms and way you may work on a bike and you'll learn that you'd need to buy a deraillier with a hanger because your bike doesn't have on built in. (this is not for you, just educational purpose)