There are two different Greek words involved.
Disciple (mathetes) means "learner" or "student." It is a term that applied and applies to all who seek to learn the way of Jesus.
Apostle (from the Greek verb apostellein) means "one who has been sent out."
The apostles in the New Testament were disciples who were understood to be specifically sent out by Jesus to extend his mission throughout the world. We might call these persons something like "chief mission strategists," those who were acting to oversee the advance of the mission of the church and to guarantee the authenticity and foundational teachings and purpose of that mission.
This focus on a ministry of oversight (episkope in Greek) is why the successors to the apostles in each place were referred to as bishops. The English word "bishop" is the Anglicized form of the Greek word episkope, meaning oversight.
So, the bishops were (and are) overseers, continuing the mission and work of the original apostles in the current day.
Watch this related Chuck Knows Church video about disciples.
This content was produced by Ask The UMC, a ministry of United Methodist Communications. First published Sept. 4, 2018.