If you're alive, you're living — in other words, you're not dead. If your apple tree blooms in the spring, you'll know it's still alive after the long, cold winter.
Living things are alive — people, animals, plants — but things can be alive figuratively as well. You might, for example, talk about keeping optimism alive or keeping the school music program alive. The word is often used to mean "spirited" or "full of energy," too, as when a birthday party comes alive just as the magician and the pony arrive. Alive comes from the Old English phrase on life, "in living" or "in life."