You humans have fingernails and toenails. Us cats, well we have CLAWS! Our claws are kind of a universal tool. We can use them to catch our pray, swipe at something to defend ourselves and do probable damage, tear meat from bones, and we can use them to help us climb trees and whatnot.
We usually have five clawed toes on our front and four on the back. My human used to have a cat that was a polydactyl; in other words, they had more than five clawed toes. The innermost claws are similar to human thumbs and are good for holding onto our prey. These are called dewclaws.
Our claws do not retract completely into our sheaths. When we are relaxed, our claws are up off of the ground. By not 'walking' on our claws, the wear and tear that would produce is reduced. This also enables us to sneak up on our prey as we can walk silently.
We are digitigrade meaning we walk on our toes. Humans are plantigrades as you walk on the soles of your feet. When you declaw us, our balance can be thrown off.
I hope you've enjoyed this insight into cats claws! Have a purrrr-fect night! MEOW!