| A molecule is chiral if it can exist in two different mirror-image forms. Your hands are chiral, for example; this is the reason you cannot wear a glove on the 'wrong' hand. However, if you turn a left-handed (the term 'handed' is generally used to refer to a particular chiral conformation) glove inside-out, you get a right-handed glove . This shows that your hands are the same shape as each other...if you could see them in four dimensions of space instead of three!
As a simpler example, consider the tetris blocks 'L' and 'gamma' :
_ _
|_| |_|
|_|_ _|_|
|_|_| |_|_|
L(left) Gamma(right)
I have arbitrarily labelled them left and right. Now, if you rotate them in the plane of the screen (as in the game tetris), you cannot superimpose one on the other. However, in 3-D space you could flip one of the shapes, and place it on top of the other, and they would match. So in two dimensions, these shapes are chiral.
The most notorious example of this is the teratogen thalidomide which caused birth defects in newborn children. Only one of the two chiral forms of this chemical is dangerous, but it was synthesised as a mixture of the two. Many drugs are ineffective in their mirror image forms - though not all drugs are chiral.
To determine the chirality of a molecule do the following:
- Find a chiral atom in the molecule. It will have four different groups attached.
- Order the groups by size (eg N > O > C > H for C[C@H](O)(N) ).
- View the central atom from the smallest group (H, in this case).
- Determine whether the other groups rotate clockwise (by size order) or not.
Clockwise rotation is R (rectus), anticlockwise is S (sinister). | 
