There are two main parts: the tread, which is the horizontal platform on which we walk (sometimes called a run) and the riser, a vertical element in a set of stairs that forms the space between one step and the next.
Risers can be closed or left open, such as what you might see on the basement or deck stairs.
There are also other terms you should know: landings, the platform between two sets of stairs--you often see this on train platforms. The nosing is the tread overhang. You aren’t required to have one, but you are required to have a tread at least 11-inches deep if you don’t.
If the two don’t follow measurement requirements, the stairs will be difficult to walk up and down, and it won’t be very safe. Risers should be at least 7-¾ inches high and the trade at least 10 to 11-inches high.
Please know, however, there may be different requirements in different parts of the country, as well as variations on winding and circular staircases. This is a big reason why you should hire a professional to do this job.
Yes, but we need to clarify repair and replacement. If a tread is broken or has any structural damage or if you suspect they need to be brought up to code, call a professional.
Sometimes replacement means overlay which is slightly smaller than the tread; when there's no structural damage, but looks faded, worn or scratched. Overlays come in a variety of materials, including naturals, synthetics, and anti-slip.
If you choose a carpet overlay and it curls over the nose of the tread, best to hire a carpet installer.