They are long and thin, and the veins run parallel to the length of the leaf. Grasses have fibrous root systems. Grass seedlings send up one rolled leaf, or spike. The growing point of grasses is at the base of the plants. If the top of the leaves is cut off, often the leaf will just keep growing as the new growth on bottom pushes the old growth higher.
New leaves come from the base of the plants.
Common grass weeds include foxtail, crabgrass, and quackgrass. It is often easier to identify individual grasses by the seedhead than by the leaves. Sedges have a triangular stem. Yellow nutsedge has a yellow-green color. Yellow Nutsedge's seedheads are very distinctive, but most seed is not viable.