It's a Cicada Killer.  Yellow and black, it is a member of the wasp family although it is not aggressive.  It burrows into lawns, nursery beds, etc., typically on the south to southeast side and in well-drained, sandy soil.  Each female digs a burrow about 1 1/2" in diameter.  The female completes and stocks about four cells in each tunnel.  cell contains from 1 to 3 paralyzed cicadas on which eggs are laid.  When eggs hatch, the larvae bore into and feed on the cicada.  The larvae overwinter in their burrows emerging the next July as adults.  Between late July and mid August, these new adults dig soil burrows, stock them with cicadas, and the cycle is repeated.  Adults die by mid September.  A large population of the Cicada Killer results in unsightly mounds of soil.  Most of the Cicada Killers seen are the males which patrol the nesting area.  They may fly about people or even hover in front of head, but they can't harm anyone.  Females do not defend their burrows and will sting only if mishandled.  Control of Cicada Killers is safely and effectively done by placing a small amount of Sevin dust down into the soil tunnel.  For a large nesting population, the area can be sprayed.  The tunnel entrances should then be left open or else the wasps may begin excavating new tunnels.