Sheila, as Mimi mentioned, Sheldon Brown's website has a wealth of information. Each major brand of bike componentry, e.g., Shimano, Campy, and SRAM, has its own pecking order, which you can determine from there respective website. As you ascend the pecking order, the components are more expensive, lighter, and arguably perform better. I say, arguably, because the performances differences that the average recreational cyclist will detect between, say, Campy Chorus and Record or Shiman Ultegra and DuraAce are debatable. I'm not saying there aren't differences, only that they may not necessarily warrant the added expense. That said, you will see differences between each company's lower- and higher-end components.
I generally agree with MT that you should try to buy the best bike you can afford--with a few caveats. First, as I've already suggested, I think the average recreational rider can and will be happy with a bike that isn't speced with top-of-the-line components. If you can afford it, then great, but I would resist thinking that a bike with lower-end components won't work for you if that's what you can afford. When budgeting for your bike, remember to factor in the cost of the other things you might want/need for the road bike, e.g., pedals, seat bag, etc. I've repeatedly said over the year and a half since I bought my road bike that the bike ends up being the cheapest part of the sport!
Second, fit and geometry are far more important than whether the bike has, say, a Shimano 105 or Ultegra rear derailleur. You can always upgrade parts--even if for a cost--but there's only so much you can do to change fit. You will ultimately end up with a "better" bike if your pre-buying research includes some things to look for with respect to fit. Be discerning when dealing with the bike shops in your area and try to find one that makes bike fit a priority. Don't assume that just because a shop is in the business of selling bikes that they're in the business of selling only those that that fit their customers. I hate to sound cynical about it, but sadly, I am.
Third, some components are more important than others so look for upgrades where they count the most, e.g., the rear derailleur and wheels. Certain other components/parts are easy enough to improve without breaking the bank. For instance, most of us end up changing out the saddle, so don't get too fixated on what kind of saddle comes standard on the bike. As an another example, better tires can also improve how a bike rides for a minor investment.
Whatever bike you buy, make good use of whatever free or discounted maintenance the bike shop offers you. The truth is that bikes--even those with top-of-the-line parts--need to be tuned now and again. It would also be to your benefit to learn some basic bike maintenance so that you can help keep your bike in good working order for less money. Again, even the most expensive bike won't run as well with a dirty chain.
Good luck, have fun with that and post pictures when you get your new bike!
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher