For the beginner choosing a mountain bike pretty much comes down to a couple of factors, what type of riding will they be doing and where do they live. The location can help narrow a decision based on the local geography. If you want to ride down steep mountain trails and all around you is gentle sloping hills a downhill bike is not going to be the best choice.
On the other hand if you do live in an area where a variety of riding styles can be done then it is up to personal preference. Do you want to ride gentle hills, climb big mountains, perform stunts, descend down difficult downhill courses, or just have fun?
Depending on your preference of riding, the bike you choose will most likely come from one of these categories: cross country, all mountain, freeride, or downhill. There will be many options to choose from within these categories, but the basic concept of the bikes within each category remains the same.
An all mountain bike is the category that makes sense for most buyers. These bikes are designed to pretty much do well for a large number of conditions. They can go just about anywhere and are light enough to do it efficiently. If general all around fun is what you are looking for, this is the type of bike to buy.
Cross country bikes may look somewhat similar to an all mountain bike but there are some big differences. They are built for speed so they are very lightweight. To go fast also means that the suspension needs to be stiffer resulting in a less comfortable ride. The cross country bike will have narrow tires to allow for higher speeds on harder surfaces. Designed to excel on hard pack surfaces this type will not be well suited for rough off road trails. For fast rides on and off road a cross country bike cannot be beat.
Freeride bikes are tough and designed to go through some rough off road terrain. Heavier than an all mountain bike it is still light enough to get you up a hill though it is going to be harder. Its tough design is made to take jumps and perform stunts as well as ride some rough off road trails.
Downhill bikes are just what the name implies, built to go downhill. They are the least common of mountain bikes you will see. They are very heavy, so going up the hills they are meant to go down is not likely to be an option. These bikes need different way to get to the top of the mountain but if you are looking for the fastest way down nothing beats a downhiller no matter how rough the trail is.
After giving it some thought, a beginner can easily come to a decision about exactly what type of mountain bike is going to be best suited for them by knowing what type of riding they will be doing.