Depending upon your budget, you'll eventually be making choices concerning frame materials, component levels and possibly suspension systems. You don't have to be an expert on these topics, but knowing the basics can help you ask important questions while you shop. The frame is the heart of the bike and affects ride and feel more than anything else. Frames come in various materials, each offering different levels of durability, stiffness and weight. Don't spend too much time fretting over frame materials.
Components are another important area on which to focus.
Components include your brakes, chain, crankset, front and rear derailleurs, hubs, shifters, seatpost and headset. Basically, they are all of the mechanisms and equipment used to transform a frame into a complete bicycle. Price-wise, manufacturers sometimes offer as many as nine levels of component groups. Component levels usually correspond to the price of the bike, but not always. The conventional thinking is that you acquire a quality frame out of the gate and then wait until your pockets are full again to upgrade components.