Grimoire Cards are unlocked from completing objectives, and uncovering new things. After your complete these accomplishments and objectives, you’ll unlock additional lore for Destiny with these cards. You can check your Grimoire progress over at Bungie.net once Destiny is released.
Does the Grimoire Card do anything else for you though? DattoDoesDestiny sheds some light on this subject in his latest video!
In other news, MoreConsole has another video showing off some rare & legendary items. If you missed the first one, check it out at the bottom of this article.