What makes Mark’s gospel different? First, it is shorter than the other Gospels. That means he doesn’t include things like Jesus’s birth and childhood. Second, Mark is action-packed, moving from one event to the next.
Third, Mark doesn’t have most of the long speeches of Jesus that Matthew records (like the Sermon on the Mount). Fourth, Mark spends six chapters (out of sixteen total) on the last week of Jesus’s life. He obviously thought it was very important.
Finally, Mark seems to reflect the perspective of the apostle Peter. Mark may have written the book based on sermons he heard Peter preach and conversations that they had about Jesus.
The main message of the book is: who is Jesus? Mark gives the answer in the first verse, but then for the rest of the book you get to watch as everyone else figures it out (or not).