Mark is the surname of John
Acts 12:12 (KJV)
12 And when he had considered the thing, he came to the house of Mary the mother of John, whose surname was Mark; where many were gathered together praying.
Acts 12:25 (KJV)
25 And Barnabas and Saul returned from Jerusalem, when they had fulfilled their ministry, and took with them John, whose surname was Mark.
Acts 15:37 (KJV)
37 And Barnabas determined to take with them John, whose surname was Mark.
Acts 15:39 (KJV)
39 And the contention was so sharp between them, that they departed asunder one from the other: and so Barnabas took Mark, and sailed unto Cyprus;
Luke was another of the many disciples.
Colossians 4:14 (KJV)
14 Luke, the beloved physician, and Demas, greet you.
2 Timothy 4:11 (KJV)
11 Only Luke is with me. Take Mark, and bring him with thee: for he is profitable to me for the ministry.
So your next question would be, if Mark is John, why a book of Mark and a book of John?
The book of Mark is John who was inspired by God to write what is written in the book of Mark.
The book of John may have been penned by John but he was not the one inspired by God to write those things in what we know as the book of John. The man inspired to write those things is know as Joseph, Joseph of Arimathaea.
Matthew 27:57 (KJV)
57 When the even was come, there came a rich man of Arimathaea, named Joseph, who also himself was Jesus' disciple: