Well, an expert I am not, but my understanding is that 'dementia' is the generic term for the condition. However, the brain is very complex and the effects we all experience can be produced in a number of different fashions, and not necessarily in the same way.
To treat dementia the doctors need to know which variant is present as medications and treatment may be different.
I've been corrected before in conversations, though I still think I am correct - in saying that Alzheimer's was originally specified as being a dementia identified in people who were not old [ie not 'senile dementia'] These days, Alzheimer's seems to be mostly identified in the older people, except for those like my Jan, who has "Early Onset Alzheimer's". I was 'corrected' by someone who reckoned that Alzheimer's was always an old persons thing.
The effects can also be caused by vascular dementia, and there are others [someone who actually know, please enlighten us on all]. Often Pick's and Parkinson's are mentioned in this context and epilepsy has also cropped up. There is also an alcohol-induced dementia.
Alzheimer's and vascular dementia have different profiles, though the symptoms of memory etc appear similar. Alzheimer's has a relatively slow decline, while vascular - being stroke induced - goes down in steps.
Many people, like your Mum and my Jan, have a combination of dementias.
Actually, often the doctors don't seem to know which one a patient has since only a post mortem can truly show. So they use Alzheimer's as a generic when talking to us, as most people have heard of that.
I always use the term dementia if possible since it covers all of them.
As usual, my caveat... the above may be total tosh, but it is my current understanding and no medic has ever bothered to fully enlighten me.
As far as I'm aware alcohol induced dementia is linked to Korsakoffs syndrome which is exactly this - dementia caused by excessive alcohol consumption.
Brucie is right in saying that dementia is the generic term for the global cognitive decline that occurs in all people with the disease.
The different names however, refer to the specific causes of the dementia. For example, alzheimers is a degenerative disease that occurs in the brain and is signified by the formation of plaques and tangles that block the signals from one part of the brain to another, whereas vascular dementia, is caused by the cutting off of blood flow to the brain (i.e linked to the heart). They all have practically the same outcome in that all people are impaired cognitively, but they are caused by different things. Think that's right!