Cats usually, but not always land on their feet. Felines are often able to land on their feet but the height of the fall is an important factor in how safe is the landing they make.
Cats have a highly-tuned sense of balance and have very flexible backbones (they have more vertebrae than humans), which allows them to twist their bodies around to right themselves when they fall. A cat’s innate ability to reorient its body during a fall is called the righting reflex, and it’s observable in kittens starting with 3 weeks old. By 7 weeks, this skill is fully developed.
When a cat jumps or falls from a high place, it uses either its sight or its vestibular apparatus (a balance system located in the inner ear) to determine up from down, and then rotates its upper body to face downward. Its lower body follows suit.
If you have cats, be careful and secure your windows! A bird or squirrel can easily distract a cat enough to cause them to lose their balance — cats can be injured in a fall, even if they can land on their feet. Sometimes shorter falls, from one or two stories, can be riskier than higher falls because the cats may not have enough time to right themselves.