When you go into a bike store to purchase a new bike, you'll find that there's no need to settle for everything that comes on the bike. It's called “fitting the bike, " and the bike shop guys will mix and match various components to give you the best ride possible, depending on your specifications.
You'll be spending a great deal of time on that saddle, so its important that it fits you well. As with any other bike component, you can get saddles for any budget, and for any need. You can get gel-filled saddles, saddles with cutouts, wide saddles and narrow ones. Women in particular need to get a saddle that fits, as their “sit bones" are spread wider apart than those of a man.
It's important that the handlebars be set at the proper height, so that the rider does not have to stretch too far forward. Handlebars are fastened onto the stem, so its quite easy to substitute a longer - or shorter - stem, in order to ensure the correct fit. Saddles, tires and brakes will cost more depending on the type you get, but the stem will be swapped out for free.
Bike tires come in different widths. Very narrow, for rode cyclists, not quite so narrow, for a cross country bike, and very fat, for the dedicated mountain biker. In addition, the treads on these tires are very important. You might want to switch out your tires depending on the terrain over which you'll be riding.
If you're going to be riding on loose dirt or mud, you'll want a widely-spaced, knobby tread pattern. This allows the tire to shed mud. If you'll be on a hard-packed, dry trail, you'll want a tire with a shallow center tread. If you don't expect to ride off-road at all, but still like those fat tires, get them without knobs, which will give you a smoother ride.
There are three types of brakes - cantilever, direct pull, and the most expensive, disc. Cantilever brakes used to be the standard, but nowadays any bike that cost over $300 will have “direct pull" brakes. These offer better stopping power. Hydraulic disc brakes are the best brakes of all - as they aren't affected by the wet, but currently they are used only on those bikes used strictly for downhill. They way more than the other types and are much more expensive.
There are two kinds of shifters - the twist shifters and trigger shifters. Some people complain that using the twist shifter tires out their hands by the end of the ride.
Pedals - Clipped or Clipless
Important for off-road bikers, as it prevents your feet from losing contact with the pedals during rough stretches. It's easier to get your feet out of toe clips than clipless pedals - which use cleats.
Once you've got your bike fitted out to your satisfaction, you'll be ready to hit the trails and have some fun!
Alastair Hamilton is a journalist who has written more articles and newsletters on this subject for http://www.bikecyclingreviews.com Find more publications about mountain bike reviews at his website.