What is the difference between an apostle and a disciple? Some people use "apostles" and "disciples" interchangeably when talking about the 12 that originally followed Jesus.  Yet, I don't commonly hear the opposite, people using those two words interchangeably when talking about those that follow Jesus after his resurrection and ascension.  When I do hear them interchanged, it doesn't sound correct.  What is the difference between an apostle and disciple?  And does it change from denomination to denomination?
The Oxford Dictionary gives a generic meaning of disciple as:


  A follower or pupil of a teacher, leader, or philosopher.


The specific meaning is given as


  A personal follower of Christ during his life, especially one of the twelve Apostles.


A definition of apostle is


  An important early Christian teacher or pioneering missionary.


Apostle is from the same Greek word as post and conveys the sense of a messenger of Christ.

Prior to the Ascension Peter etc. were essentially following  Jesus and learning from Him. They were disciples. After Whitsun they were teaching about Him. They were His apostles. So the fundamental difference is between learning and teaching.

They also conducted a preaching tour during Jesus' earthly ministry. So at that point they were both.

In practice, though, people often use the terms interchangeably.