The Legends

The legends surrounding Saint George are very varied. One of them concerns the famous dragon, with which he is invariably portrayed. According to legend, a pagan town in Libya (possibly Salem) was being terrorised by a dragon. The locals kept throwing sheep and cattle to it to placate it, and when the animals got scarce and the dragon still remained unsatisfied, they started sacrificing some of the citizenry. 

The local children were picked in a lottery, and finally the lottery fell upon the local princess named Cleodolinda, who was to be thrown to the beast, but Good Saint George came along, promised to release the citizens from this terror.  He made the sign of the cross and wounded the dragon using his lance and sword and then rescued the fair princess. The princess is then said to have bound the dragon with her girdle.  The dragon was taken back to the town square where George slaughtered the beast with his lance and some say beheaded it.

At this the townsfolk converted to Christianity. 
The origin of the legend, which is very well known, came originally from the way in which the Greek Church honoured George. They venerated him as a soldier saint and told many stories of his bravery and protection in battle. The western Christians, joining with the Byzantine Christians in the Crusades, elaborated and misinterpreted the Greek traditions and devised their own version. The story we know today of Saint George and the dragon dates from the troubadours of the 14th century.

The legend is said to represent Christianity (George) saving suffering humanity (Princess) over Evil (Dragon), 
probably derived from this extract of the Bible.

"And the great dragon was cast down, the old
serpent, he that is called the Devil and Satan, the
deceiver of the whole world; he was cast down to
the earth, and his angels were cast down with him.
And I heard a great voice in heaven, saying, Now
is come the Salvation, and the power and the
kingdom of our God and the authority of his
Christ."
Revelation 12:9-10

Nb. Everything quoted is under investigation for error.  Any input appreciated.