An apostle is one who is sent by another as an envoy. As such, apostles of Jesus are those that he sends to represent him in the world. As such, Jesus himself could even be considered an apostle of his Father in Heaven (Hebrews 3:1).
More commonly, Apostle refers to followers of Jesus.
Often the terms disciple and apostle are used interchangeably, although some would say that a Disciple is one of the original twelve followers, whereas an Apostle is a more generic term that applies to all followers of Jesus.