Do masks behave differently than selections?

I am trying to understand if there is a difference between masks and selections, other than that the masks are permanent.  Selections are easy to use as masks, but if you close the application they go away (annoying!).  I have noticed that live brushes do not bleed outside of selections, but masks do.  I am also having a hard time finding explanations of mask linking and flattening, and whether masks can be made to apply to multiple layers.  I am very new to Fresco, but love it.  Unfortunately, advanced technique information seems very hard to find.