Sarcasm is an ironic or satirical remark tempered by humor. Mainly, people use it to say the opposite of what's true to make someone look or feel foolish. For example, let's say you see someone struggling to open a door and you ask them, "Do you want help?" If they reply by saying, "No thanks. I'm really enjoying the challenge," you'll know they're being sarcastic. Sarcasm is all about the context and tone of voice, which is why it works better verbally. It's something you'll know when you hear it.
