To add to DVap's accurate info, nicotine is asymmetric. Like your hand. Your hand has no plane of symmetry where everything on one side of the plane is mirrored with everything on the other side. Your right hand is the mirror image of your left hand, and they cannot be perfectly superimposed one on the other, with all fingers matched and pointing in the same direction. Molecules can have the same type of asymmetry. D- and L-nicotine are the same molecule when drawn in two dimensions, but mirror images when drawn in 3-D. Chemically they are equal in most situations. But they are not equal when they are interacting with cell receptors, which are also asymmetric.
Much like if someone wants to shake hands with me with their right hand, the best shake will be me using my right hand. My left hand can be used, but it will not be the same full grip. But what is "right" and "left" at the molecular level?
If I make a solution of L-nicotine, it will rotate plane-polarized light shown on it to the left, that is, counter clockwise. We call this "levorotatory", or "L" for short. A solution of D-nicotine will rotate plane-polarized light to the right, clockwise. This is "dextrorotatory", or "D".
(+) and (-) are just another way of saying "D" or "L".
D- and L-nicotine are examples of enantiomers, or non-superimposable mirror-image stereoisomers. We say that nicotine is chiral, meaning "handed".
"S" and "R" are absolute molecular descriptions, and are based on the orientation of atomic groups around the chiral center, which is the asymmetric carbon. "R" means "right", but the R-enantiomer does not have to be the "D-" enantiomer. In other words, how a molecule rotates pp light does not necessarily dictate if it is R or S. "S" is the mirror image of the "R" enantiomer. "S" stands for "sinister", as in the left hand is, in the old way of thinking, the sinister hand.
L-nic is the enantiomer with the highest bio-activity, and its the enantiomer that tobacco makes. As DVap said, extraction is the cheapest and best way to get and purify it, as in pharma-grade. D-nic is wildly expensive and not what we would want anyway. Even a mixture of the two enantiomers can only be made synthetically and will still be far more expensive than just extracted L-nic. It's very nice that the cheapest is actually the best, in this case. Its not nice, however, when erroneous claims about these enantiomers are used to try to sell juice, and hopefully this is not occurring any more.
