Wheels are made up of:
Rim: The metal hoop that the tyre attaches to and usually forms a braking surface.
Hub: Found at the centre, or hub, of the wheel. This is where cassettes or sprockets attach and the component that attaches to the frame.
Spokes: Spokes link the hub to the rim.
Nipples: Adjust the tension of the spokes. Rim, nipple and spoke
If you notice that you have a broken spoke or the wheel is bent then consult a qualified bike mechanic.