Annuals are weeds that seed, flower and seed in same year, e.g. barnyard grass, foxtail, pigweed, lamb's-quarters, ragweed. Winter annuals germinate in fall, usually as a rosette, and flower and seed the following spring, e.g. peppergrass and chickweed. In strawberries, some weeds can be both annual and winter annuals. Pull up the weeds to see what the roots are like. Annuals generally have small tap or fibrous root systems.
Biennials generally have a taproot.
An important identification feature is the ligule. A ligule is an appendage on the inner side of a grass leaf at the point where the sheath joins the blade. If the ligule is absent, the weed is barnyard grass, if the ligule is hairlike or fringed, you have foxtail. Crabgrass has a membranous ligule. Nutsedge is not a grass but often mistaken for a grass.