Grubs live and feed in the soil. It's easy to miss them as they gradually cut the roots out from under your lawn until brown patches begin to appear and the grubs are finally discovered.
Pull back the turf if you suspect grubs. If the lawn pulls up easily (like new sod), you may find white grubs in the top inch or so of the soil.
Grubs are the larval (or worm) state of many types of beetles. The beetles lay their eggs in your lawn, and the newly hatched grub work their way through the thatch and into the soil, where they feed on roots of grass plants. Most beetles lay their eggs mid to late summer, and the young grubs do their greatest damage during the fall months.
As the weather cools, most grubs burrow deeper into the soil for the winter. They then return to the surface to feed again as the soil warms in the spring. After this spring feeding, the grubs pupate into adults beetles and being the cycle again.