query_posts() is overly simplistic and problematic way to modify main query of a page by replacing it with new instance of the query. It is inefficient (re-runs SQL queries) and will outright fail in some circumstances (especially often when dealing with posts pagination). Any modern WP code should use more reliable methods, like making use ofpre_get_posts hook, for this purpose. ( don’t use query_posts() ever)
get_posts() is very similar in usage and accepts same arguments (with some nuances, like different defaults), but returns array of posts, doesn’t modify global variables and is safe to use anywhere;
WP_Query class powers 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.