Weed control products fall into one of two categories: pre-emergent and post-emergent. Pre-emergents prevent seeds from germinating so the weed can never get started. Post-emergents kill weeds that have already sprouted.
For organic gardeners the best available pre-emergent is corn gluten meal. For post-emergent control, Marshall Grain recommends
20% Horticultural Vinegar
AgraLawn Crabgrass Control