Emacs modes are simply libraries of Lisp code that extend, modify, or enhance Emacs is some way. The distinction isn't the easiest thing to grasp until you've worked with a few of them off and on, but let's give it a shot. Only one major mode can be active at a given time.
Many minor modes can be active at a given time.
Major modes tend to be language or task-specific, while minor modes are smaller and less specific utilities that cut across many tasks. There's a mode that I use quite often when I'm writing plain old text files. It's called text-mode. It understands how to identify words and paragraphs and generally makes sure that it does what I expect when I use the normal navigation keystrokes. When I'm writing text for human consumption, I typically want it to look good.