The higher the contrast ratios the blacker blacks appear and the whiter whites appear. Plasmas are generally considered to have better contrast ratios than LCDs.
Don't believe a word about constrast ratios - everyone measures them differently and alot of people confuse display contrast, contrast ratio and dynamic range. Add this to the fact that numerical values are often meaningless since it generally doesn't correspond well with the eye's own sense of visual contrast.
LCD's actually have a better dynamic range than plasma's