To puke means to vomit or throw up: when your stomach forces its contents back up through your mouth. It's an informal, blunt word that kids often use instead of more polite terms like “vomit” or “be sick.”

You might puke after eating something spoiled, when you have the flu, or from motion sickness on a twisty car ride. Your stomach contracts powerfully, and everything comes rushing back up. It's unpleasant and messy, but it's also your body's way of getting rid of something it thinks is harmful.

The word puke can also be used as a noun for the stuff itself, though that's probably more detail than anyone wants. People sometimes use it as an exaggeration too: “That neon green paint color makes me want to puke” means it's so ugly it's almost physically revolting, not that it would actually make someone sick.

While puke is acceptable in casual conversation, you'd probably use “vomit” or “throw up” when talking to your doctor or writing a formal explanation for missing school.