"According to the Synoptic Gospels, Jesus referred to himself as "Son of man" in three contexts, each with its own circle of fairly distinct meanings. He used this self-designation of
(1) his earthly work and its (frequently) humble condition (e.g., Mark 2:10, 28 parr.; Matt 11:19=Luke 7:34; Matt 8:20=Luke 9:58);
(2) his coming suffering, death, and resurrection (Mark 9:9,12; Mark 14:21 and, above all, Mark 8:31; 9:31; 10:33-34 parr.);
(3) his future coming in heavenly glory to act with sovereign power at a final judgement (e.g., Mark 8:38; 13:26-27 parr.; Matt 24:27=Luke 17:24; Matt 25:31-32; see John 5:27).
These classifications show how the "Son of man" served as a way of indicating Jesus' importance and even universal relevance. This was especially true of the class (3) sayings. In other words, "Son of man" was used to say what Jesus did rather than what he was. It was not and did not become a title in the normal sense — at least not on the lips of Jesus himself". (taken from http://en.wikipedia.org/wiki/Son_of_man_(Christianity)).
Also remember that this term appears 107 times in the Hebrew Bible.