Sage's are generally the promoted versions of mages. These units all contain the ability to wield magic, but can also wield other items depending on which game that you're playing.
In Fire Emblem: Path of Radiance, mages are allowed to choose whether they want to learn how to use a knife, or if they would rather learn how to use staves.
Without a mage, your team isn't complete. Despite their low defense, these units are immensely important to your team, and can bring victories much easier then without.
 Strengths and Weaknesses
Mages tend to dominate Knights and Laguz. They're allowed to strike from both near and afar, so they're never not allowed to strike back. When it comes to knights and other slow moving units with low resistance, mages can strike a huge vulnerable spot in the enemy. They're not only able to attack without being countered (if they're attacking a one ranged unit when they're a 1-2), they can make a huge wound in that enemy. With Laguz, it's the same. If the laguz isn't in its transformed self, then your mage will most likely take the enemy down in one or two hits. Fire harms beast laguz, wind harms bird laguz, and thunder harms dragon laguz.
However, these mages aren't perfect. Their defense is almost always way to low, and if they're hit by a strong force, they're most likely dead. With the right skills and level, this can be avoided, but keep in mind that they're not meant to be the center of attention. Keep them back, and away from harm.
 Important Mages
Related ThreadsBetter Arch sage - last post by @ May 17, 2008
Who is best sage? - last post by @ Aug 12, 2007
Bishop vs. Sage - last post @ Jun 24, 2014
Mage-------------Sage/Mage Knight - last post by @ Jan 1, 2009
whats better? sage w/ dark magic, or a super pupil - last post by @ Aug 5, 2008