Kyle writes in about this simple arduino and bulldozer kit robot that follows a projected line:

A regular line follower robot consists one to three photodiodes that reads brightness (usually with IR lights) on the floor. Based on the readings, we program it in a way that the center photodiode always stays on the black line on the floor. If either side of the photodiodes reads black, that means the robot is off the track, turns the robot the other direction so that the robot will always stay in the center.