Grubs are the larvae of certain kinds of beetles, such as masked chafers and June beetles. Grubs can typically be found in your lawn during the late summer months after the beetles have laid their eggs around June. By feeding on the grassroots, grubs begin to kill patches of your lawn, leaving it wilted and brown in certain areas.
As the weather starts cooling in fall, grubs will burrow into the soil for protection from cold winter temperatures. In the spring, as the soil warms, they will return to the surface to begin feeding again. After this spring feeding, the grubs pupate into adult beetles and the cycle begins again.
To tell if your lawn is infested with grubs, pull on the area of turf in question. If the turf comes up easily like new sod, check the soil. If grubs are present, you should easily see them.