The term "weed" refers to a plant deemed unwanted by someone, somewhere. Grasses popping up in a garden plot may be considered weeds by a horticulturist, while those same prized garden plants popping up in a grassy pasture may be considered weeds by the farmer. In natural areas, weeds are often called invasive species.
One of the most common and invasive weeds in Davis is Bermuda Grass.
UC Davis is very active in weed research. A useful site with weed information is its Weed Research and Information Center.