As advised in another thread, I'm going to be buying my first road bike through a LBS, but in the meantime I've been surfing the 'net and trying to become a more knowledgeable rider.

Naturally, I took a look at BikesDirect. I'm obviously much too green at buying bikes to take advantage of the deals on BikesDirect, especially seeing how their site categorizes bikes in part by their components! Further adding to the newbies confusion, it seems that some of the names (e.g. Shimano Tiagra) apply both to drivetrain and braking components.

Is there any online resource that provides a good overview of the various commonly available components by manufacturer and grade? I can understand what I'm buying in terms of fit, comfort and the general enjoyment of how a bike rides, but I have absolutely no clue how to evaluate the quality of the components that might be installed on the bike.

Thanks!