A phrase is said in sarcasm but is followeed by a sharp contrast with the phrase "NOT". This joke can be said either seriously or with obvious sarcasm. Often times the humor in the joke is in how obnoxious the "NOT" sounds. A 'not joke' is intended to put a person on the spot and embarrass him. It should be used when someone is really proud of something they've done. There is a pause between the statement and following "NOT". It was made popular by the movie Borat.
My 4 year old cousin was doing a puzzle. When he was done, he came up to me and said "look what I done mikey" I said "oh that's really impressize" 'pause' "NOT!!" He started crying and ran away. Needless to say it was hilarious and I laughed hysterically. My not joke worked perfectly.
A not joke is a joke from the early to mid 90s that was told in many cases in grade school. Part of the reason they were discussed in borat is that anybody who was around then remembers that those jokes haven't been used much since then. They were not "made popular" by borat.
"Not-jokes" were made popular long before the Borat movie, namely in the movie "Wayne's World" from 1992.
Filled with pop culture references, the sketches and film started catchphrases such as "Schwing!" and "Schyea", as well as popularizing "That's what she said", "Party on!" and the use of "...Not!" after apparently affirmative sentences in order to state the contrary (as used by the metal band Anthrax in the late 1980s and early 1990s). This form of joke became known as "not joke".