A common modern shorthand makes Apollo the sun god, but early Greek religion did not. The sun was Helios, a distinct deity who drove the solar chariot. Apollo's domains were prophecy, healing, plague, music, archery and the disciplining of the young. The identification of Apollo with the sun is a later development, driven by philosophical and allegorical readers from roughly the fifth century BCE onward and cemented in the Roman and Neoplatonic periods; it is a real strand of the tradition, but it is secondary, not original.
The Iliad opens with the other Apollo entirely. Slighted through his priest, he comes down "like night", and his arrows spread plague through the Greek camp for nine days. The same god who sends disease is Paian, the healer who can lift it, and later the father of Asklepios, the god of medicine. This pairing, the archer who both inflicts and cures the sudden affliction, is close to the centre of who Apollo was to the Greeks.
He is also the god of the lyre and of measured order, set against the ecstatic, wine-loosened worship of Dionysos. Greek thought used the two as a permanent contrast between form and frenzy.