Usage is the act of using something. Your cell phone usage is something you'll want to keep track of so you don't find yourself surprised by overages and extra charges that might make you want to cry.
Usage can also refer to the usual and correct way a language is spoken or written. If you want to make sure you're using the right word or using a tricky word correctly, you should consult a book on English usage. Usage can also mean an accepted practice or custom. An anthropologist might describe the specific time-honored usages that a particular tribe observes whenever a baby is born.