Apollo is to most of greek texts the god of the sun. Helios was the god of the sun during the time of the Titans. Before the Dorians first took control of Greece all the greecians beleived in Helios. Hence the city Helos was built in his glory. When the Dorians brought the religion of the Olympians to Greece by force the city was renamed to Apolloniki. In honor of the Dorian god of the sun Apollo.

Apollo some would say, others might say helios. So which is it? I always believed apollo to be the god of music and poetry. It's a little confusing because it's almost as if apollo took helios' position and poor helios now forgotten.