What is the difference between amnesia, alzheimer’s and dementia?

Question

Everytime I hear about these three diseases I always put them in the same box: forgetting stuff. And I never really understood the difference between them

In: Biology

Answer 1

Amnesia – you forget everything at once, but it wears off.
Alzheimer’s – you forget everything backwards. (You remember the past but not today.)
Dementia – you forget how to do things and the things around you don’t make sense.

This is an incredibly simplistic explanation of the three.