Electronics will form the brain of the robot, it will control what it does and how it receives information. The micro processor you use will be the brain, but it will need other components to work, whether it be pull up resistors for use with IO, or LED's for simple output. Some robots will not use a micro-processor, but will perform by using simple electronics, but that is a whole topic of its own which we will skip for the time being. For more information of resistor values, click here to use the resistor colour code utility.