A reminder helps you remember something. If you write "don't forget lunch" on your arm every morning, it means you need a reminder to grab your brown paper bag on your way out the door.
A reminder helps you remember, whether it's an appointment, an acquaintance's name, or a grocery list. You can give yourself a reminder, or write one to another person who's prone to forgetting details. The word's been around since the 1650s, and it came from remind, which originally meant "to remember." Later, remind came to mean "to make someone remember," or "to put someone in mind of something."