The Gospels of Matthew and Luke are made up from Mark’s earlier gospel, hypothetical Q (a posited source used by both Matthew & Luke), and material unique to each gospel (L for Luke, M for Matthew). Once Mark (since both Luke & Matthew used much of Mark’s gospel for their source material) and Q are taken away we are still left with a substantial amount of material in each gospel. This material (not to be found in either Mark or Q) is what is referred to as unique material. Both Luke & Matthew have this unique material that could have once been a series of sources that were written or oral, or a mixture of oral and written sources.
What is quite interesting, however, is the connection between Luke’s unique material, L, and the Gospel of John. For example, it is clear that both L and John agree that there were two apostles named Judas (Luke 6:16; John 14:22), both suggest that the act of betrayal occurred because Satan entered Judas, making Judas less responsible (Luke 22:3; John 13:2), both record the episode in which the slave of the high priest has his ear cut off at the arrest of Jesus (Luke 22:50; John 18:10), both record Pilate denying Jesus’ guilt on three occasions (Luke 23:4, 14, 20; John 19:4, 6, 12), and both are aware of a close relationship among Mary and Martha and Jesus. This is quite a substantial amount of correspondence between John and L. It is also important to note that John, as most scholars believe, did not use the synoptics as sources, which would include Luke’s gospel within which L rests.
I think that it is probably the case that a large portion of the special Lukan material was part of a tradition with which John was also familiar. This would suggest that John made use of tradition(s) that predated his gospel, probably lending credence to the fact that he did not make narratives (at least the ones he mentions that match with L) up out of whole cloth, which is promising for those interested in the historical veracity of the gospel accounts (of course this would assume the historical nature of the traditions Luke picks up that John later uses).