Pre-existing arguments when using queries

I am struggling to understand how queries work with get_posts(), query_posts() and WP_Query().
In particular I do not understand what pre-existing selection may exist when using these on a given page template.
From various blogs seems that the default situation is that query_posts() and WP_Query() apply any pre-existing filters whilst get_posts supresses them.
However the codex says query_posts() ignores other parameters it receives via the URL (such as page number or category.
Are these filters / parameters the same or something different?
How can I definitively tell what pre-existing selection will exist when I create a query?
PLEASE HELP!