Okay, Merriam Webster says:
1: characterized by pretension: as
a : making usually unjustified or excessive claims (as of value or standing) <the pretentious fraud who assumes a love of culture that is alien to him — Richard Watts>
b : expressive of affected, unwarranted, or exaggerated importance, worth, or stature <pretentious language> <pretentious houses>
2: making demands on one's skill, ability, or means : ambitious <the pretentious daring of the Green Mountain Boys in crossing the lake — American Guide Series: Vermont>
The second definition is what people acting in a way described in the 1.a and 1.b think they are doing. Pretention, the way I understand the term, is all about striving to be what you are really not. The desire to be more artsy or more intellectual than you actually are in your heart of hearts. It is teenage poetry. It happens to great artists as well. I love Woody Allen, but when he tries to copy Bergman, he fails and comes across as pretentious. Then again, the attempt that miserably failed in the eyes of one person might be a success in the eyes of the next, so who's to say? But that's what I think it comes down to. You are pretentious when you aim at somethink you consider of high (intellectual or other) value and fail in the attempt without noticing and actually feel superior about it.