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.
Key verse about Apostle: The apostles gathered around Jesus and reported to him all they had done and taught (Mark 6:30, NIV).
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 700-page website ABibleADay.com to encourage people to explore the Bible. His main blog and numerous books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.