These things go by various names officially, things like "cat wall climbers" and "cat wall shelves". Personally, I refer to them as "cat ski slopes".

Pictures:

DIY:

Note: Despite some people's focus on things being "shelf"-like, these things DON'T need to be level. Cats friggin' know how to climb trees, and tree limbs are never level. My favorite setups are where multiple cats play on several different "shelves", and the "shelves" are sloped.