Answer:
The Synoptic Gospels are Matthew, Mark and Luke. They are called synoptic because they are intriguingly similar and intriguingly different.
The most common theory is that Mark is the oldest of these, and Matthew and Luke in various ways used Mark along with other material to create their individual narratives.
Scholars believe that this other material consisted of material unique to each of Matthew and Luke (ie material they wrote themselves - called 'M' and 'L') and further material that is common to both Matthew and Luke but not found in Mark. This material consists mostly of the sayings of Jesus and scholars now believe that this came from a now lost source called 'Q' (from the German 'Quelle' meaning 'source').
Other theories - for example, that Matthew (or Luke) is the oldest Gospel and Mark wrote his account later leaving out chunks of material was once a common belief, but modern textual criticism suggests that Mark was the first gospel to have been written with additions made by Luke and Matthew.