Gospel - the 'Good News' and taken to be eye-witness accounts.
We have three consistent 'eye witness' accounts in Matthew, Mark and Luke and because of this we often refer to them as the 'synoptic' Gospels: (Same) 'syn' - 'optic' (sight) and use the shorthand // (meaning can be found in the other synoptics) and one of a different kind in John.
Each of the synoptics bear the name of their author - that's obvious innit?
Each of the synoptics was written for an intended audience and their assumed knowledge shapes the content of each of them.
Matthew - Written for a Jewish audience: Begins with genealogy to show Jesus fits the bill with regard to fulfilling the Old Testament prophecies and continues to defends his deity as the Christ.
Mark - Written for a Gentile audience. Explains Jewish customs and explains various terms. Quotes Old Testament less than Matthew or Luke.
Luke - Written for intellectual Hellenistic (Greek) Jews. Also writes the book of Acts - this is the 'home of the parable' and seeks to make sense of that 'which has been taught' - meaning to help the Jews understand that Jesus is the fulfilment of their hope.
And then there's the 'Good News':
John - Written for everyone, this is the theological and evangelistic Gospel and is the first home of truly Christ centred (Christocentric) writing - the home of miracle and the 'I AM' statements (7 of each) - not a parable, genealogy or baptism to be found here!
There are loads of theories about how the synoptics came to be written - I grew up with the four source theory (I know, I know) and regardless it's a great place to start thinking about how our Bible came to be written. So in a nutshell (should that be a nutcase?):
There was a common source, 'Q' (from the German 'Quelle' - source) to which Matthew and Luke added their own observations 'M' & 'L' (a bit like someone reading something about the 1966 World Cup final and two comanpanions adding their reminiscences). Mark is a 'standalone' Gospel and so we have three Gospels looking at the same situation (birth, life, death, resurrection) coming out of four sources.
C'est voila - the four source theory diagram (and explanation)
My Fourth Day of Christmas gift to you :-)