A nail is the pointed piece of metal that you pound with a hammer when you're building something. When you go to a nail salon, however, don't bring a pack of nails; bring your toes and fingers instead!
When construction workers pound nails into wood, they nail them. Another kind of nail is the one at the end of each of your fingers and toes. In addition to hitting a nail, the verb version of nail also means "to arrest," "to succeed easily," and "to hit." Nail polish is what you use to color the nails on your hands (or feet), and when you "hit the nail on the head," you get the exact right answer.