Mages are the traditional caster class, using a wide array of magic to control the battlefield. The 4 mage skill trees are all focused on using a different element – Spirit, Storm, Inferno and Winter. Mages can specialize as Rift mage, Necromancer, or Knight-Enchanter.
The storm tree focuses on various lightning-themed spells. Some skills like Chain Lightning are essentially unchanged from the previous DA games, but there are also a lot of cool new skills like Stormbringer and Static Charge. The Lightning tree is good for damage and crowd control.
Inferno spells are fire-based abilities that deal high damage and also include several skills that cause enemies to burn and panic. The Fire tree is good for burst damage (with skills like Fire mine) as well as effective CC with Fear.
The Winter tree uses frost abilities to slow and freeze enemies, making it ideal for a control-oriented mage. This tree is also good for survivability, with skills like Fade Step.
The spirit tree is a defensive tree that uses spiritual power to protect yourself and aid your allies with spells like barrier, revival, and passive mana regeneration. This tree is ideal for a support-oriented mage.