What is culture jamming?
Culture jamming is what you make it...but to be technical, here is a defenition according to Wikipedia, not too bad:

Culture jamming is the act of transforming existing mass media to produce negative commentary about itself, using the original medium's communication method. It is a form of public activism which is generally in opposition to commercialism, and the vectors of corporate image. The aim of culture jamming is to create a contrast between corporate image and the realities of the corporation. This is done symbolically, with the "detournement" of pop iconography.

It is based on the idea that advertising is little more than propaganda for established interests, and that there is a lack of an available means for alternative expression in industrialized nations. Culture jamming is a resistance movement to the perceived hegemony of popular culture, based on the ideas of "guerrilla communication".

so if this is something that "floats your boat"...take it for a ride. have fun. be safe, and always remember to be courteous ;)

for inspiration check out these websites:

billboard liberation front
Adbusters