Stairs or steps are one way to get between unlevel ground or floors. They can be carpeted, wood, or tile and often extend the floor covering. They can be indoors or outdoors, although outdoors staircases are oftentimes smaller and only reach a porch or patio.
Steps have risers, the vertical part that goes up. (Sometimes left out to create a more "open" style.) The tread is the flat, horizontal part that you step where you put your foot.
There are various standards about the ratio of the tread to riser, the angle of the pitch (from the edge of one step to the next), and formulas for the size of steps to make them safe and comfortable to use. The "normal" rise of the pitch is an angle of about 30 %. A lower pitch is more expensive because it takes up more horizontal space and requires more steps, but it can be easier for older or disabled users.
Some stairs curve or spiral around. The steps are narrower on one side than the other -- called winders. Others have a landing platform between sets. They may have a turn at the landing.
- Wikipedia's article on stairway