What is the difference between dementia and alzheimer's disease?

Alzheimer's is one of several forms of dementias. It is progressive, while some of the others are not.

I don't think there is solid knowledge on avoidance. Some say that mental exercises such as Sudoku puzzles and learning new languages can be helpful. There are also suggestions that nicotine, but NOT smoking is also helpful. Both ideas might be nothing more than wishful thinking.