A vapor barrier is also known as “poly” or "moisture barriers." Most building codes now require vapor barrier in the crawlspace. 40% of moisture comes from the ground, therefore, the plastic is putting a barrier between the moisture that comes from the earth and the sub floor of your home. Poly is intended to retard the migration of moisture in your crawlspace.
Any vapor retarder that is not reinforced refers to the thickness of the plastic. Anything that is reinforced beyond 12 mil refers to the thickness of the fiber. The vapor barrier is reinforced to make it stronger so that it won't rip while you are moving around in your crawlspace.
12 Mil is a stronger plastic and will last longer. The difference in color you see from various 12 mil vapor barriers on the market only differs by who manufactures it and has no practical application. When installing 12 Mil, you should always lay the side with color on the ground and leave the white side facing up.