If you look at Wikipedia's list of St Johns, you will find 61 saints with the name John. The index of saints provided by Catholic Online lists even more saints with the name John, 126. So who is is our Saint John?
Unfortunately we have to start by pointing to a confusion, namely that St John the Evangelist (wikipedia; Catholic online) and St John the Apostle (wikipedia; Catholic online) are two names by which the same Saint – our St John – is traditionally known. But the two names are often kept distinct – and wikipedia also has an entry on St John of Patmos, who some say is the same person – because there are some uncertainties in the known history. Our St John should not be confused with St John the Baptist, after whom many churches are named.
Traditionally, John was one of the group of 12 men called to follow Jesus, known as his disciples. He was the younger brother of James, and together with Peter these three appear to have been closest to Jesus. In spite of much persecution, which eventually claimed the lives of both James and Peter, John appears to have been able to live to old age, latterly in the city of Ephesus, before seeing out his final years living in exile on the nearby Greek Island of Patmos. The date of John's death is around the year 100 AD.
John has traditionally been identified as the New Testament author of the Gospel of John, the three epistles (letters) I John, II John and III John, and the Book of the Revelation, the final book of the New Testament. But this is where the confusion arises, because some scholars have claimed – and not without some merit – that these books are not from the same author. The wikipedia article on the authorship of these books is a good starting point for anyone interested in these issues.
What we know about John comes from two main sources. First we have the Gospels themselves, where we get the impression that John was a relatively young man when following Jesus. Second, John is one of the few of the disciples for whom we have a chain of followers through early history. Polycarp, Bishop of Smyrna (ca 60–155) was a disciple of John, and his disciple was Irenaeus (Bishop of Lyon, France, second century AD). Writings of both men have been preserved to this day.
St John's day is 27th December, which is sufficiently close to Christmas for it to be easy to overlook.
In the images below, there are a number of traditional points to note. First, the symbol of St John is an eagle, which was commonly used as an image at the start of medieval gospels. Second, there is a tradition that the Romans tried to boil John to death in a cauldron of oil, but he survived unharmed and was subsequently banished to Patmos instead. There is also a story of John being given a chalice of poisoned wine, from which the poison emerged in the form of a snake as John blessed the chalice. Many images of St John mix these images together with him as the author of the gospel.