Modern Catholic Dictionary:
APOSTLE. A messenger and authorized representative of the sender. Broadly used in Scripture, it refers to many followers of Jesus who spread his message. More precisely, however, it applies to the original twelve men chosen by Jesus to be his immediate aides. They are referred to as disciples during the period in which he was instructing them, but following his ascension they are always called Apostles. After Pentecost they spoke and acted with confidence and assurance in teaching others what he had taught them and in assuming leadership roles in the early church. They were ordained priests by Christ at the Last Supper and were commissioned by him to preach the Gospel to all mankind (Matthew 28:19-20). (Etym. Latin apostolus, an apostle; Greek apostolos, one who is sent off.)
DISCIPLE. One who is learning or has learned. In the New Testament the word describes any follower of Jesus’ teaching (Matthew 10:1). During his public ministry it referred as well to his twelve chosen aides, but in the Acts of the Apostles they are always referred to as Apostles (Acts 1:26). (Etym. Latin discipulus, pupil, follower.)