Dementia vs Alzheimer's disease?

ALZ falls under the Dementia umbrella but it effects the brain much differently.

The difference was discribed to me as such.

Dementia...the person knows its a stove but has forgotten how to use it.

ALZ...forgets its a stove. Take two fingers from each had and cross them like tic tack toe. The brain has memory pathways. Now take one of those fingers and lift it off the finger its laying on. This is a disruption in the pathway. They become tangles. Only an autopsy can really determine ALZ.

Dementia is neither a disease nor a diagnosis. Dementia refers to symptoms usually caused by some underlying disease most often a neurodegenerative disease like Alzheimer's, to name only one. However, there are also treatable conditions that can mimic true dementia symptoms. That's why it's very important to get an accurate diagnosis of the cause of the dementia symptoms. I use a simple analogy of a headache. It is neither a disease nor a diagnosis. It is a symptom that may be caused by stress, the flu or a brain tumor. You would want to know the cause of your headache. So to with dementia.  

Good overview here:

https://www.mayoclinic.org/diseases-conditions/dementia/symptoms-causes/syc-20352013

https://www.alz.org/alzheimer_s_dementia

"There are over 100 forms of dementia, with the most well-known form of dementia being Alzheimer's disease."

Going to refer you to your favorite search engine.
Type into search "Different Dementias" or any similar words.
You will be supplied with a list.
Alzheimer's Disease is a particular KIND of dementia, and was named for the man who discovered aspects of the disease.
Dementia is the overall word for many differing kinds of dementia.
Wikipedia has a good short definition for you that starts out with:

"Dementia is a syndrome associated with many neurodegenerative diseases (most commonly Alzheimer's), which is characterized by a general decline in cognitive abilities that impacts a person's ability to perform everyday activities. This typically involves problems with memory, thinking, behavior, and motor control.[10] Aside from memory impairment and a disruption in thought patterns, the most common symptoms include emotional problems, difficulties with language, and decreased motivation.[2] The symptoms may be described as occurring in a continuum over several stages."..........

So looking up "define dementia" will get you this short and quite complete definition of dementia.

When I first began in nursing over 50 years ago there were no differing diagnoses. You, as an elder, either were competent or were "senile". That antiquated word has now bit the dust and we know there are many different kinds of dementia, each with its special characteristics and each diagnosed more or less easily. Just a few: Frontotemporal dementia, Lewy's Bodie Dementia, Alzheimer's dementia, and so on and on.

Welcome to the Forum, and I wish you luck; there's a ton of stuff to research.

Dementia is an umbrella term, under that umbrella fall diseases such as Alzheimer's, vascular dementia, lewy body dementia, white matter disease, fronto-temporal dementia and many others.