The type of hardware needed to build a robot is quite diverse. It really depends on what kind of robot you're trying to build. (How heavy is it, what does it need to do, the material that would be best suited, the quantity needed, etc.) While many different types of metal are common material for the parts, it's not always the best choice for certain functions, given it could be too heavy or too expensive, so in some cases, plastic or rubber are used, the latter especially if it's supposed to have wheels. Whatever the case, it has to be sturdy in built, not just in the part alone, but how it fits with the others in order for it to work properly.
Gears are considered by many to be the symbol of not only robotics, but machinery and technology in general, and for good reason. It symbolizes how doing one thing leads to other events happening, while the first action continues. Not only are gears common in robotics, there are different variations of it that can be used. Of course, if you're going to talk about gears, you might as well include chains, pulleys, and belts because not only can they prove just as important, but also in conjunction in gears as a driving mechanism. But when it comes to the power source, what powers the driving mechanism, a power supply is needed for that. Once again, it depends on several factors, (how big is the robot and how hard is it going to need to work) so a proper source could range form a few batteries to being plugged into an outlet at all times.
And finally, there's the interface, or the point where human interaction decide what the robot does next. It could be a set of buttons or a connected wire to a computer that holds the program to run the robot, it's a part in the robot's design that hopefully, over time plays a smaller part in how it operates.