--the R-lipoic acid is composed entirely of the right-handed isomer (shape) of the lipoic acid molecule, which is the one that is active in the body.

Alpha lipoic contains both the left and right-handed isomers, so one is actually paying somewhat for something the body can't absorb. The ability of manufacturers to isolate the right-handed isomer is fairly recent.

Because R-lipoic contains only the active isomer, one can use it in considerably smaller amounts than alpha lipoic to get a similar effect, which helps those who find large doses of alpha lipoic too acidy. A ratio of 1:6 is often cited--100mg of R-lipoic is roughly equivalent in effect to 600mg of alpha lipoic.