From Wikipedia:
There is much more on wikipedia. I recommend reading it for more information on this topic.The final twelve verses, 16:9-20, are not present in two fourth-century Greek manuscripts, the Sinaiticus and Vaticanus.[17] The only other Greek manuscript without the ending is a twelfth century commentary on Matthew and Mark, known as "304".[18] As such, verses 9-20 are present in 99% of the Greek manuscripts,[19] with most of these witnessing to the "Byzantine text-type" manuscript tradition.[20] However, Sinaiticus and Vaticanus, largely of the Alexandrian text-type, are generally highly valued as a witness to the autograph of Mark due to their antiquity. Because of this, and also because of linguistic and stylistic differences between these verses and the rest of the Gospel, there has resulted scholarly debate concerning the authenticity of these passages and the original ending of the Gospel.
Codex Washingtonianus (late 4th or early 5th century A.D.), included the addition to 16:14 that is known as the Freer Logion.
Another ending, called the shorter ending is found in an Old Latin manuscript of Mark.[21] Some seventh-to-ninth-century Greek manuscripts and Syriac, Sahidic, Bohairic and Ethiopic have minor variations on this shorter ending.
Hypotheses on how to explain the textual variations include:
Mark intentionally ended his Gospel at 16:8, and someone else (at an early date) wrote the concluding lines.
Mark did not intend to end at 16:8, but was somehow prevented from finishing (perhaps by his own death), whereupon another person finished the work.
The Gospel originally contained a different (perhaps similar) ending that was lost, for one reason or another, whereafter the current ending was added.
Verses 16:9-20 are authentic, and were omitted or lost from the Sinaiticus and Vaticanus for one reason or another, perhaps accidental, perhaps intentional.
James H. Charlesworth pointed out that Codex Syriacus (a 5th-century translation), Codex Vaticanus (mid-4th century), and Codex Bobiensis (4th- or 5th-century Latin) are all early manuscripts that exclude the Marcan Appendix. In addition to these, approximately 100 early Armenian translations, as well as the two oldest Georgian translations, also omitted the appendix (James Bentley p. 179) In Secrets of Mt. Sinai, James Bentley made this observation about the omission of the Marcan Appendix in Codex Sinaiticus: "The scribe who brought Mark's Gospel to an end in Codex Sinaiticus had no doubt that it finished at chapter 16, verse 8. He underlined the text with a fine artistic squiggle, and wrote, "The Gospel according to Mark." Immediately following begins the Gospel of Luke (p. 139).
