The typical lawn consists of many different plant species, some valued by the homeowner as lawn grass and others less desired and regarded as weeds. It's not always easy to tell the difference. In caring for your lawn, you need to learn to identify what species comprise it and decide which of those should stay and which should go. Weeds both detract from the appearance of a lawn and compete for nutrients and water.
The terms "weed" and "grass" overlap considerably at times, since grasses are often considered some of the peskiest weeds, according to Turfgrass Science, produced by the Purdue University and University of Illinois extension services. Grass describes a type of plant with long blades and hollow stems. Weeds are defined subjectively and include any plant you don't want that competes with desirable plants.
Aside from grasses, weeds take other forms. Broadleaved weeds have wide leaves with branching veins. Sedges grow upright, a lot like grasses, but have solid, three-sided stems. Grasses can also be counted as weeds when you don't want them in your yard.
You need to be able to recognize and describe the features to identify grasses and distinguish one species from another. The sheath is the base part of the grass that surrounds the stem, and the blade emerges from it. As for the other parts, the ligule, when present, attaches the sheath to the blade, and the collar is where they meet on the outside of the blade. Rhizomes and stolons are lateral stems connecting individual plants, underground and aboveground, respectively, enabling them to spread.
If the weed is a grass, proceed as you would when identifying a desirable species. If it is a broadleaved weed, describe the number, shape and texture of the leaves, as well as the root and the flowers, if present. When identifying sedges, describe the seedhead and note the color of the tuber -- an underground structure used for storing nutrients -- if it has one.
Once you have the descriptive details, use a lawn-care book or weed identification key to find the species that matches the grass or weed you're trying to identify. North Carolina State University offers TurfFiles, an online resource that allows you to select plant characteristics and provides possible grass and weed matches. If you still can't figure out what you have, your local extension office can help.