Can anybody explain the concept of tripthongs?
And why are they not there in american accent?
The "i" in "site" sounds like "s-a-i-t". The best thing to do is look up the word here and listen to it: www.answers.com. Just click on the speaker. Another example of a diphthong is the long "o" sound, as in the word "know". The "o" sound in "know" is immediately followed by a quick "u" sound. So the word actually sounds like this: n-o-u, but the "u" sound is quick. Failure to produce diphthong sounds such as this contribute to a foreign accent.
Another type of diphthong includes two vowel sounds in which the first vowel sound glides into the second vowel sound, as in the word "noise".
I would look here and here for triphthongs.
triphthong A vowel sound in which the vocal organs move from one position through a second to a third.
There are no triphthongs among the English phonemes, but such sounds occur when a closing diphthong is followed by /ə/. At least, they theoretically occur in a careful pronunciation of such words as player ...