Unlike computers, robots can move around and interact with the world much like a player can. They can not interact with external components, however! If you need to communicate with a computer or other robots, use a wireless network card, or create some low-level protocol using redstone signals via a redstone card, for example.
Robots are built by placing a computer case of any tier in an assembler. Higher tier computer cases can build more complex robots, due to being able to hold a higher tier CPU. Complexity of the robot (as shown in the assembler) is determined by the tier of the components and upgrades placed in the robot slots; higher tier components will increase the complexity more than a lower tier component. If the complexity of the robot is too high, the assembler will not build the robot.
Various upgrades can be placed into robots to increase the functionality. These include inventory and inventory controller upgrades, tank upgrades, navigation upgrade, among others. Upgrade and card containers can be placed in the robot for on-the-fly insertion and removal of upgrades and components. A disk drive can also be placed inside a robot to allow floppy disks to be inserted, which will let you install openOS to the robot (an alternative is to install openOS to a blank hard drive using a computer, and using the pre-installed hard drive as a component in the robot assembly).