A step has two faces. The tread is the horizontal board you put your foot on; the riser is the vertical board (or open gap) between one tread and the next. The nosing is the front edge of the tread that projects past the riser below it. The stringers underneath carry both — see stair stringer rise and run for how the overall layout is worked out.
The IRC (R311.7.5) sets the dimensions for homes: risers no taller than 7-3/4”, treads at least 10” deep measured from nosing to nosing, and no more than 3/8” difference between the tallest and shortest riser, or the deepest and shallowest tread, in a flight. Where treads are less than 11” deep, a nosing of 3/4”–1-1/4” is required. Open risers are allowed, but when the stair rises more than 30” the openings must not let a 4” sphere pass through — so a child can’t slip between steps.
Tread material depends on where the stair is. Exterior and deck stairs usually use two 2x6s or a 5/4 decking board pair (spaced for drainage) over stringers no more than about 16”–18” apart. Interior stairs use solid 1”–1-1/8” hardwood treads, often oak, over a 3/4” riser board. When figuring riser heights, remember to add the finished flooring at the top and bottom — a stair that is perfect in framing can end up with an illegal first or last step after flooring goes down.