R/S is called Racemic.
The R and S isomers refers to chiral centers of a molecule. Like it is a mirror image of one another, such as your right and left hand. They are similar but different, and extend on the same plain of one another, but there is much further discussion on stereochemistry in the ADD. Basically if you look at your hands, your fingers are like the positioning of elements on say a phenyl ring, all of the elements are the same on this ring except the positioning (1,2,3,4 etc) is different.
There are also Enantiomers (mirror images of a chiral molecule) that are similar to what I was just mentioning but the molecules don't "match up". Often with Enantiomers only one isomer has the most desirable effects, but with other substances one isomer is dangerous for the fetus where as another is perfectly safe.
As an aside, you will also note D/L vs. R/S as well.
(+) (-) relates to the plain of polarized light, clockwise(+) vs. counterclockwise (-)