Homeowner lawns to farmer fields, these guys cover them all. This species-group stands
ready to mess up just about any habitat you might want to be weed-free.
Large crabgrass is larger, more hairy, and possibly adapted to drier habitats than smooth crabgrass. Large crabgrass is often called hairy crabgrass. Smooth crabgrass is more likely to be home lawns, not to mention your favorite golf course.
Both species have a membranous ligule.
Compare the collar region in these two related species:
Left picture: large, left; smooth, right. Right picture: smooth, left; large, right. If you get confused with this just follow the hairs to large crabgrass.
Below you can see one of the really neat adaptations both crabgrasses possess: the ability to root at its stem nodes (left). Large crabgrass has what is called a decumbent habit, meaning the stems get lazy and grow horizontally instead of straight up. If the node is in contact with the soil it can root at that stem node. This is a handy adaptation to have if you get your roots ripped off by tillage or inter-row cultivation. Given enought moist soil you might be able to re-root your stem after the experience.
The genus of this species-group is Digitaria, meaning digits as in fingers. Do you see why some clever taxonomist thought of this name when you look at the seedheads of this species-group below?:
The spikelets on the digitated seedhead:
Crabgrass reeking havoc in this corn field: