The most common discolorations on carpets are stains, which occur when a substance comes in contact with the carpet and it gets embedded into the fibers of the carpet. A very good example is if you spill a cup of coffee on your carpet, it is going to leave a stain.
Since a stain is actually embedded into the fibers, there is potentially more of a problem with getting rid of it, as it can become a permanent problem if it is not taken care of. Stains occur in fibers that have empty dye sites. That is,in carpets that are dyed in the post production phase. If the carpet is dyed while the fibers are still in liquid form, then the dye goes all through the fiber, then there are no empty dye sites, and they are more difficult to stain.
If you do suffer a stain on a carpet, the best way to handle it is to immediately blot the liquid, whatever it may be, with a dry towel and then treat the area with a stain remover, or a homemade solution of a drop of clear dish washing liquid in a cup of warm water.
A spot on the carpet may look like a stain, but is not always from the result of a spill of something. A spot can be the result of dirt or another substance being left behind due to grease or oily substances being left behind. Spots are merely minor points of residue that are left behind and are fairly easily cleaned up with standard means.