An available Lane Keep Assist [1] system applies steering torque to help the vehicle stay in the center of the lane (provided of course you're on a road with clearly marked lines).
To help keep you from drifting out of your lane when cruise control is in operation, an available Lane Departure Warning System gives an audible warning. After this warning, it takes corrective steering action to help keep you in your lane. When cruise control is not engaged, the system sounds a warning if you drift out of your lane.