Reading a few bike magazines will give you an idea. Often department stores or mass merchants will put good parts on a cheap frame to make the bike look like you're getting extra value. When you need replacement parts, it's great when you get back on the bike right away. Enjoy your bike and maintain it regularly. An overstretched chain can really damage your drive train, then you'll have to replace all your sprockets.
Preventative maintenance is the key to a safe and healthy bike.