The book provides feats, prestige classes, and other options for characters interested in magic. Unlike Complete Arcane, Complete Mage contains no new core classes, but it contains alternative class features for existing classes.
Andy Collins explains how this book relates to Complete Arcane: "Both books cover similar topics--how to enhance your character's magical (arcane) abilities. That said, they don't follow exactly the same format (though both contain spells, feats, and prestige classes as one might expect), so the range of subject matter varies a bit. Complete Mage, for example, doesn't introduce new classes like Complete Arcane did, though it does provide some new options (feats, spells, and so on) for the new classes from Complete Arcane."
Shannon Appelcline identified Complete Mage as one of the books that "changed the way that D&D worked in dramatic ways" and may have influenced the design of 4th edition D&D.