Basically, anything foreign to the construction of the fiber. Where do we find the soil?  Most soil will be at the entrance. One study showed it takes 15 steps or about 45 feet.   Walk off mats both inside and outside will help with keeping the soil contained.

Proctor and Gamble did a study and found that 79% of soil is insoluble.  This soil is best removed with agitation and vacuuming.  This insoluble soil is not only difficult to remove wet but also damages the carpet with its sharp edges causing a carpet to ugly out.

10% of the soil was found to be water soluble. The remaining 11% was oily.

So, if it is not insoluble, it is likely either acidic and/or oily. Most foods are acidic and oily. Oils in are carpets are from organic sources and are made up of fatty acids. We can deal with oily acid soil by using the right kinds of solutions to remove it.