Harmonization. When the scribe knew information from a parallel passage, he would sometimes bring the material from the other verse into the current verse. This most likely was done out of respect for the Bible, the scribe not wanting to leave out any biblical data. Even if you believe that a certain passage was harmonized and view part of the verse as an addition, you haven't lost any biblical teaching since the phrase does exist in the other gospel.
NA Novum Testamentum Graece by the Deutsche Bibelgesellschaft (current edition is 28th revised), and is often referred to as Nestle-Aland (or NA), named after two of its main authors. Nestle-Aland lists some 30,000 variants with only the major manuscript evidence for each variant
UBS The same text in the NA is also published as The Greek New Testament by the United Bible Societies (currently in its fifth revised edition). The UBS shows 1,408 variants with a much fuller list of manuscripts behind each of the variants.
Textus Receptus (TR).
Codex D. Codex Bezae ...
P66. Papyrus 66 ...
P75. Papyrus 75 ...