Mages specialize in the casting of spells, but they're competent gunfighters as well. Mages are versatile spellcasters and can use a variety of spells, from standard offensive attacks, through character buffs, to defense and healing.
Contrary to many works of fiction, mages aren't "squishy" or weak in Shadowrun and can go toe-to-toe with the toughest opponents if sufficiently skilled. However, with the wide range of spells available to Mages/Shamans, it is best for them to specialize in a particular branch of magic. A master of one is better than master of none. Like shamans, mages must forgo extensive use of cyberware as cyberware drains their essence, hampering their ability to cast spells. Also, note that Conjuring spells should not be confused with Spellcasting spells.
Leylines are useful to a mage/shaman. However, the increased power to spells can be a double-edged sword; a mage's offensive spells often do not discriminate between friend or foe.
It takes less karma to be a good mage, as compared to being a good shaman.