‘Now a certain man was sick, Lazarus of Bethany, of the village of Mary and her sister Martha, and it was that Mary who anointed the Lord with ointment and wiped his feet with her hair, whose brother Lazarus was sick.'
Lazarus lived in Bethany, about two miles outside Jerusalem. He was ‘brother to Martha and Mary'. It may well have been at their house that Jesus tended to stay when He was in Jerusalem (Matthew 21:17; Mark 11:11; Luke 10:38-42 - see also John 12:1-8), and it could well have been their ass on which He entered Jerusalem (Mark 11:1). He is constantly shown as having a close relationship with the family. They were His friends.
The fact that Lazarus was identified by his relationship to Mary and Martha demonstrates that the author expected the latter to be well known to his readers. This confirms that the Gospel was written against a background of known material. As they appeared in Luke 10:38-42, this may suggest that he knew that his readers would be familiar with the tradition behind Luke's Gospel.
John 11:2 is also interesting. The incident John mentions has not yet been recorded (see John 12:1-8). This again suggests that John expected that that incident was also well known to his readers, even before he himself wrote about it. Thus it is clear that John, in writing, depended on the fact that his readers already had a solid background of knowledge about the life of Jesus gained from the tradition, something possibly known as ‘The Testimony of Jesus Christ' (Revelation 1:2; Revelation 1:9; Revelation 12:17; Revelation 19:10; compare 2 Timothy 1:8). Note how it is paralleled with ‘the Word of God' i.e. the Old Testament Scriptures (Mark 7:13), not however seen as just a written record but as a living witness.