Jehovah's Witnesses are Christians. (See What Does It Mean to Be a Christian?)
Some have used an artificial, trinity-specific definition of the term "Christian" which excludes anyone who does not believe that Jesus is God Himself, rather than the Son of God.
Yet even though Jehovah's Witnesses do not believe that Jesus is God Himself, they still believe in Jesus, they believe that he is extremely important and they follow his teachings. Even disinterested theologians and secular dictionaries acknowledge that Jehovah's Witnesses are a Christian religion. (For much more, see Jehovah's Witnesses a Christian Religion.)
Jehovah's Witnesses do not believe in the trinity because
(1.) It has pagan origins:
"The trinity was a major preoccupation of Egyptian theologians .... Three gods are combined and treated as a single being, addressed in the singular. In this way the spiritual force of Egyptian religion shows a direct link with Christian theology." - Egyptian Religion. (For much more, see Trinity And Pagan Influence.)
(2.) It is not found in the Bible and developed gradually over several centuries after Christ:
"Neither the word Trinity, nor the explicit doctrine as such, appears in the New Testament, nor did Jesus and his followers intend to contradict the Shema in the Old Testament: 'Hear, O Israel: The Lord our God is one Lord' -- Deut. 6:4
. . . The [trinity] doctrine developed gradually over several centuries and through many controversies . . . By the end of the 4th century . . . the doctrine of the Trinity took substantially the form it has maintained ever since." -- Micropædia, Vol. X, p. 126. (1976) - The New Encyclopedia Britannica. (For much more, see History of the Christian Trinity.)
(3) Rather than using the entire Bible as context, many believers in the trinity rely only on a few selected, so-called 'proof-texts' which do not hold up under closer scrutiny. (For much more, see Trinity 'Proof - Texts'.)