Io, the volcanically active innermost large moon of Jupiter, was a target of intense study during the recently completed NASA Galileo mission to Jupiter (1989-2003). Galileo's suite of instruments obtained unprecedented observations of Io, including high spatial resolution imaging in the visible and infrared. This paper reviews the insights gained about Io's surface, atmosphere and space environment during the Galileo mission. Io is thought to have a large Fe-FeS core, whose radius is slightly less than half the radius of Io and whose mass is 20% of the moon. The lack of an intrinsic magnetic field implies that the core is either completely solid or completely liquid. The mantle of Io appears to undergo a high degree of partial melting (20-50% molten) that produces ultramafic lavas dominated by Mg-rich orthopyroxene in an apparent 'mushy magma ocean', suggesting an undifferentiated mantle. The crust of Io is thought to be rigid, 20-30 km thick, cold away from volcanic heat sources and composed of mafic to ultramafic silicates. Tidal flexing due to Io's orbital resonance produces ~100 m tides at the surface, generating heat that powers Io's volcanism. Silicate volcanism appears to be dominant at most hot spots, although secondary sulfur volcanism may be important in some areas. The key discoveries of the Galileo era at Io were: (1) the detection of high-temperature volcanism (ultramafic, superheated mafic or 'ceramic'); (2) the detection of both S2 and SO2 gas in Ionian plumes; (3) the distinction between eruption styles, including between Pelean plumes (originating from central vents) and Promethean plumes (originating from silicate lava flow fronts); (4) the relationship between mountains and paterae, which indicates that many paterae are formed as magma preferentially ascends along tectonic faults associated with mountain building; (5) the lack of detection of an intrinsic magnetic field; (6) a new estimate of global heat flow; and (7) increased understanding of the relationship between Io, its plasma torus and Jupiter's magnetic field. There is an apparent paradox between Io's potentially ultramafic volcanism (suggestive of a primitive, undifferentiated mantle) and the widespread intensity of the volcanism on Io (which should have produced a volume of lava ~140 times the volume of Io over the last 4.5 Ga, resulting in more silicic materials). The resolution of this paradox requires either an Io that only recently (geologically) entered its tidal resonance and became volcanically active or a response of Io's lithosphere-mantle to tidal heating that has in some way prevented extreme differentiation. Understanding this problem is one of many important issues about Io that remain unresolved. We conclude this paper with a discussion of the types of future observations, from the ground and from space, that will be needed to address these issues.