A chiral molecule is the chemical term for a molecule that is not symmetrical - that is a molecule who's mirror image is not super imposable. The example that is universally quoted to explain this concept is the left and right hand. If you look at your left and right hands they have the same number of parts as each other and if you put them side by side they are the mirror image of one and other, as you can see in the first picture. But as soon as you try to superimpose or line up the two mirror images, they don't align. You can see in the second picture how I have tried to align both of my hands. While I can line up the index fingers, none of the other fingers align meaning that my hands are not symmetrical. It doesn't matter which way I rotate the two images they will never superimpose. As Wikipedia explains 'this difference in symmetry 
becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand'. An achiral molecule is one that is symmetrical and does not differ from its mirror image and so is super imposable. Pure Skin Junkie gives the example of the letter A. The mirror image is identical to the original and when you try to superimpose the two they line up completely.Ok, I can feel your eyes starting to glaze over already. Get on with it, Sarah. What does any of this have to do with skincare I hear you ask? Well, whilst some skincare ranges contain only naturally derived ingredients, the majority are comprised of laboratory created ingredients. It is much easier and cheaper to synthesise Vitamin E, for example, in a laboratory than to try and extract it from natural sources. Vitamin E can be found in almonds, sunflower seeds, wheat germ oil, amongst other things but the quantities of these ingredients required to acquire sufficient Vitamin E, and the cost associated with extracting it would make the final skincare product exorbitantly expensive.
When molecules are synthesised by man they always form in pairs that are the mirror image of each other. So, like your left and right hand, these mirror imaged molecules contain all the same parts but are not identical to one and other. Each mirror image of the molecule is given a prefix to its chemical name – either d or l. So, using the Vitamin E example, the chemical name for Vitamin E is alpha-tocopherol so the mirror images of the molecules are called d-alpha-tocopherol and l-alpha-tocopherol. But even though these molecules are nearly identical (the only difference being the mirror image) only one of the molecules can be used by the human body – in this case the molecule with the d-prefix (d-alpha-tocopherol). So chirally correct skincare contains ingredients that have been tested to ensure that they only contain the active molecule, out of the mirror imaged pair, that can be used by the human body. The molecules are sorted so that only the active molecule is added. With Vitamin E it has already been determined that d-alpha-tocopherol the active molecule so any Vitamin E that is added to chirally correct skincare will have been sorted so that only d-alpha-tocopherol is added. If your skincare is not chirally correct, and the vitamin E molecules have not been sorted, it will be listed in the ingredients as dl-alpha-tocopherol, as it contains a 50/50 mixture of the d-alpha-tocopherol and l-alpha-tocopherol molecules.
So what happens if the molecules are not sorted? Generally nothing; however, your skincare will be less potent. When a molecule is made, equal amounts of the l-molecule and d-molecule are created. So, if your skincare range claims to have 10ml of Vitamin E in it, but the Vitamin E has not been sorted to get the chirally correct molecule then your skincare will actually contain 5ml of active Vitamin E and 5ml of a molecule that does absolutely nothing - so your skincare is half as potent as you expect. In rare cases, one of the mirror images of the molecule might actually be dangerous. One of the best known examples of this is Thalidomide. Thalidomide was a sedative drug given to pregnant women to ease morning sickness, that also ended up causing shocking birth defects. Unfortunately, when the Thalidomide molecules were created, the scientists didn't test the effects of both of the mirror images of the molecules. So one of the mirror images of the molecule was the sedative, but it was the opposing mirror image that caused the birth defects. Had both of the molecules been tested for efficacy in the product development stages then the whole Thalidomide tragedy could have been avoided, but instead, the women were given the unsorted 50/50 mixtures of both Thalidomide molecules. Now that scientists are aware that the effects of each molecule can be so drastically different, all new molecules are tested. Therefore, there is no reason to be concerned if you are using skincare that is not chirally correct that it could be doing you damage, just be aware that it may not be as potent or effective as you think.
I translated the majority of the information in this blog from the following websites:
http://en.wikipedia.org/wiki/Chirality_(chemistry)
http://en.wikipedia.org/wiki/Tocopherol