John Mark was the cousin of Barnabas. He joined Paul and Barnabas on their first missionary journey, but returned home in the middle of it (Acts 13:13). As a result of this, when Barnabas wanted John Mark to accompany him on a second missionary journey with Paul, Paul refused. This disagreement resulted in Paul and Barnabas parting ways (Acts 15:39). Later, however, it appears as if there was a reconciliation seeing as Paul commends Mark to others (2 Timothy 4:11; Philemon 1:24).
24 and so do Mark, Aristarchus, Demas, and Luke, my fellow workers.