The remainder of something is whatever is left over. As a noun, remainder refers to a part, number, or quantity that is left.
For example:-
I couldn't eat all my Subway sandwich, so hubby ate the remainder.
A reminder can be anything that causes you to remember something, it can also be a message or note, or any communciation designed to ensure that you remember something.
For example:-
I set my Google calendar to send me a reminder before an English session begins. |
