Some resources that have comprehensive weed identification charts are the Scotts Lawn Company, the University of Maryland College of Agriculture and Natural Resources, and Penn State College of Agricultural Sciences. The Scotts Lawn company has charts for both broad leaf and grassy weeds.
Broad leaf weeds are easy to tell from lawn grass, and their different physiology makes it easier to manufacture and use herbicides that kill them without harming the grass. Grass-like weeds are tougher to eradicate because many of them are closely related to the desirable grass in the lawn. Indeed, some of these weeds, such as ryegrass and bluegrass, are used as turf grass.
A truly comprehensive weed identification chart also has links that detail the type of weed the gardener is researching. The links at the Scotts Lawn Service pages, for example, take the viewer to pages that help him identify the weed and find the Scotts products that can control it.
Besides unwanted types of turf grass, grassy weeds include sedges, timothy, sandburs, witchgrass and wild barley. Crabgrass is also a famous pest, as are panic grass and redtop. The easier-to-spot broad leaf weeds include types of plantain, dandelion, speedwell, curly dock and sheep sorrel.