I can understand why the question was asked as the two names are often used interchangeably.
Dementia is not a disease but a set of symptoms which are a feature of many diseases, perhaps the most common being Alzheimer's.
There's a lot more than can be said but that is the basic difference.
It is very confusing that 'Alzheimer's' is used so often when actually 'dementia' is far more appropriate.
I was reading the blurb for the Alzheimer's Show which is coming to Manchester (isn't that's a bit odd, by the way, a 'show'?) and is described as 'the UK's only dedicated exhibition and conference for families and professionals caring for a person with dementia'. So why isn't it called 'The Dementia Show'?
A very unhelpful and unnecessary confusion, I would suggest.