The Story about Might
Might is your strength, currently it will increase your base swing damage and also your vita attack.
Each point of Might also increase your Vita and little bit of Mana, Might is primary attribute for Warriors, secondary for Poets and tertiary for Mages and Rogues.

The Story about Grace
Grace is your charm, unlike Warriors, Rogues get more benefit from it, As it is primary attribute for Rogues, it is increase your base swing damage and also your vita attack. Rogues also have increase dogde rate from Grace, this dogde rate also apply to other class but at lower rate. Each point of Grace also improve your hit rate/miss rate of the melee attack type, it does increase your Vita and Mana as well. Grace is secondary attribute for Mages and Warriors, tertiary for Poets

The Story about Will
Will is your magical power, thus it is Primary attribute for Mages and Poets, it is increase your magical damage and fail chances of your magic. Each point of Will increase your Mana and a little bit of Vita. Will is secondary attribute for Rogues and tertiary for Warriors.