Memoirs

I am taking a memoirs class. What are memoirs? When does one write memoirs? What is the difference between an autobiography and a memoir? Questions are what I have after my first class. Reading, and a lot of it is what I must do. And writing, writing, and more writing. First a letter for next week to someone about something using scene setting. Making it a letter to someone about something sensitive and/or meaningful. I don't know who I will write to as of yet but I have several candidates in mind.
And memoir writing; I will have to start that on paper although I have so many thoughts scurrying through a maze in my mind.