An ablative CO2 laser rays, when applied to a skin surface, will evaporate water from the skin cells and hence destroy the cells on entire treated area, effectively removing the whole superficial layer of the skin. This will have a very dramatic and aggressive effect on the skin with prolonged down time and slow recovery. However it will very effectively treat your deep lines or deep scars with minimal number of treatment applications required.
The Fractional CO2 laser is using this ablative (evaporating) effect on only a fraction (a part) of the treated skin in a patterned, pixel-like, way. By doing so, it leaves, very small, untouched skin parts, which will allow skin to heal a bit faster and have less down time post laser treatment. However, to achieve similar to the ablative laser treatment effect, you will need to have a series of treatments.