Whenever you say something in response to someone, you reply. If your teacher asks why you're late to school, you might reply, "Sorry, I missed the bus."
To answer back or respond is to reply, and whatever you say is also a reply. Your reply could be a long quotation from a Shakespeare soliloquy, or it might be a simple, silent nod of the head. You can also reply using a letter, a text, or an email. Reply comes from the Late Latin replicare, which means both "to reply or repeat," but also "to fold back."