What's the difference between do and does?

Excuse me, I'm curious about the distinction between "do" and "does." Could you please explain the difference in their usage and function in the English language? I've noticed they're both used to form questions and affirmations, but I'm having trouble pinpointing the exact difference. Perhaps you could provide some examples to illustrate their distinct purposes? I'm really eager to grasp this concept and improve my grasp of the English grammar. Thank you in advance for your help!