Spots are temporary in nature, typically found on the tips of the fibers, greasy/oily in nature. When professionally cleaned, they are totally removed.
Stains are permanent in nature. Usually identified by bright colors. Some mustards, colored spices, Kool-Aid, wines, spaghetti sauce, furniture stains, make-up, paint, etc. are ofen lightened in color, but not completely removed.
Spills come from liquids (soft drinks, coffee, vomit, urine, etc). The liquid has gone beyond the carpet fiber, backing, and into the padding. Due to wicking (capilary) action after cleaning, the residue may wick back up to the tips of the fibers. Special treatment to these areas can reduce the chances of their return.
Filtration is that unsightly dark deposit found along the edges where the baseboards meet the carpet. Air currents inside the home cool and drop or deposit pollution and soot-like deposits at the base of the walls or under door openings. These complex chemical deposits require special attention to remove. If not periodically removed, permanent discoloration can result.