When should you use WP_Query vs query_posts() vs get_posts()?

• query_posts() should be used in one and only case if you need to modify main query of page. It sets a lot of global variables and will lead to obscure and horrible bugs if used in any other place and for any other purpose;
• get_posts() is very similar in mechanics and accepts same arguments, but returns array of posts, doesn’t modify global variables and is safe to use anywhere;
• WP_Query class power both behind the scenes, but you can also create and work with own object of it. Bit more complex, less restrictions, also safe to use anywhere.