A diphthong or "gliding vowel" occurs where there are two adjacent vowel sounds in the same syllable. They occur with most English speakers in the words hay, out, and boy, for example.
I'm no phonetician, but as a Canadian I'm familiar with the phenomenon called Canadian raising -- our English speakers will often "raise" the diphthong before a voiceless consonant such as t or k. That is, we give the second vowel sound in the diphthong more prominence (with volume or pitch). Strangely, the common stereotype is that we say "aboot" rather than "about" but in actuality, many have observed that most Canadians pronounce it like "aboat" (as in, what you'd call the canoe in today's photo). And, of course, there are numerous regional variations!
The about/aboot/aboat discussion can be quite controversial, since some Canadians avow that they have no discernible accent. Me, I like to ramp mine up while travelling and throw in a few extra "eh?"s. Americans are always very helpful and friendly when approached by a silly lost Canadian!
Don't forget the first H in diphthong: dipthong* is a low probability typo on the Ballard List.
(Canoe off the shore of Toronto photo from Wikimedia Commons)