Hylian Knights are basically rolemodel members. They are expected to be active, make good posts, obey the rules, be respectful to members, etc. They also have more exact requirements like rep, post count and how long they've been a member. They're actually chosen by the current Knights nominating and voting on members.
Nobles are kinda similar, just more so. They also represent the site. Only longtime Knights can become Nobles, and they're nominated and voted on by current Nobles in much the same way.
Hope that helps! ^^