What exactly defines a Main Loop and a Secondary Loop?

What exactly defines a Main Loop and a Secondary Loop?

My assumption is that the main loop is always the first loop and you're
always supposed to use query_post? Is this correct? Then whenever you you
need to feed in some arguments to get a different set or order of posts
you use WP_Query or get_posts. These would both be secondary loops? I just
feel I'm hardly interacting with the Main Loop at all so I have this
feeling, whilst everything works, I'm just not doing things properly.