Tradition attributes authorship of the Gospel of Mark to a companion and secretary to St. Peter, named in the book of Acts as John Mark (12:22, 25, 15:37.) For the record, I don't buy the Modernist invention of a "Q" source for the Gospels. I accept the traditional understanding: the Gospels were written in the order they're published in the Bible, and by the authors whose names appear on them. The Apostle Matthew wrote Matthew, the Apostle John wrote John, and so forth.
The Gospel of Mark appears intended for an audience of Roman Christians. Mark begins with Christ's baptism in the River Jordan, glossing over the infancy narratives and genealogies present in the other Gospels. He explains the definitions of Jewish terms that would be unfamiliar to Gentile audiences. Healings, miracles, and exorcisms performed by Christ proportionally take up more of Mark than any other Gospel. Since the beginning, the Catholic Church has symbolized St. Mark with the lion. The angel of St. Matthew, the bull of St. Luke, and the eagle of St. John along with Mark's lion are associated with the creatures described in Revelation 4, and the six winged Seraphim which surround the throne of God.
Tradition holds that St. Mark established the Coptic Church of Alexandria in Egypt. He is also the patron saint of Venice, Italy. From the Novus Ordo Mass for the feast of St. Mark, 1 Peter 5:5-14:
Clothe yourselves with humility
in your dealings with one another, for:
God opposes the proud
but bestows favor on the humble.
So humble yourselves under the mighty hand of God,
that he may exalt you in due time.
Cast all your worries upon him because he cares for you.
Be sober and vigilant.
Your opponent the Devil is prowling around like a roaring lion
looking for someone to devour.
Resist him, steadfast in faith,
knowing that your brothers and sisters throughout the world
undergo the same sufferings.
The God of all grace
who called you to his eternal glory through Christ Jesus
will himself restore, confirm, strengthen, and establish you
after you have suffered a little.
To him be dominion forever. Amen.
I write you this briefly through Silvanus,
whom I consider a faithful brother,
exhorting you and testifying that this is the true grace of God.
Remain firm in it.
The chosen one at Babylon sends you greeting, as does Mark, my son.
Greet one another with a loving kiss.
Peace to all of you who are in Christ.