There are many different types of carpet. Most of them are nylon. The biggest manufacturer of carpet is Shaw. You can get low-cut pile, medium pile, or high pile. One thing to remember when purchasing new carpet is, if you are buying a shag carpet, you want to get one with a high density. The higher the number, the more dense it is. For instance, if you buy a loose, white shag carpet, it is going to be very hard to keep clean and maintain, as the strands are loose and the dirt, dust, stains, allergens, and other foreign material will bond to the carpet fiber. You want to buy a dense shag carpet if you can afford it.
They also make berber. This is an excellent grade of carpet. You can trash this carpet, and it will clean up very nicely, as the dirt, dust, stains, etc. stay on the top of the carpet and cannot bond very easily to the fibers. This carpet is more expensive but should last a lot longer.
You also want to invest in a good carpet pad. Most retailers who sell carpeting are knowledgeable about pads and can help you pick the right one for your budget.
You want to vacuum your carpet weekly so that the dust, dirt, etc. cannot bond to you carpet fiber. Think of carpet like a tee shirt, if you wear your tee shirt for a year and never wash it, when you do wash it, much of the dirt and stains are not going to come out. Vacuuming your carpet is like washing your tee shirt. You want to stay on top of this task if you want to get the longest life out of your carpet. Also remember to have your carpet professionally cleaned at least once a year to prolong its life.