Magic Barrier reduces magic damage taken by 50% for its duration. This includes defense against all offensive sorceries, offensive miracles, and any weapon that deals magic damage. Magic Barrier counts as a personal buff, so it is impossible to use other buffs (such as Power Within, Flash Sweat, etc.) while it is in effect.
Great Magic Barrier is an upgrade over Magic Barrier, boasting higher magic resistance in exchange for fewer casts.