Runes- How do they work

Can someone explain to me how runes work, I found one in an item once, but I didn't know what to do with it. This may be n00bish or whatever, but I honestly haven't a clue how they work.

[PS: I'm level 12]

apply it to a piece of armor by double clicking on it than clicking the piece of armor

must be a general rune or your primary profession rune

apply it to a piece of armor by double clicking on it than clicking the piece of armor

must be a general rune or your primary profession rune

http://www.guildwarsguru.com/rune-listing-id29.php