Who was Chrysostom? I often hear him mentioned. Was he an important figure in church history?

John Chrysostom was born in Antioch (347), became a solitary monk (379-381), and then a leader in the Antioch church (381-397). His moniker "chrysostom" means golden-mouthed—referring to his eloquent preaching. His approach was more common-sense than the highly figurative interpretation of scripture of the Alexandrians. Around 600 sermons and 246 letters of his survive, plus a commentary on Acts. He was also popular because he opposed extravagance on the part of church leaders.

Chrysostom eventually became bishop of Constantinople (398), was banished (403), and exiled to Armenia (404). He died in Anatolia in 407.

For more, the Wikipedia article is quite good.