Meme

The term Internet meme is used to describe a concept that spreads virally.
A Meme is often a saying or joke, a rumor, an altered or original image, a complete website, a video clip or animation, or an offbeat news story.
An Internet meme is an inside joke, that a large number of Internet users are in on, these are very popular on sites such as 4chan.
Internet memes have a tendency to evolve and spread extremely swiftly, sometimes going in and out of popularity in just days. They are spread organically, voluntarily, and peer to peer, rather than by compulsion, predetermined path, or completely automated means.

The term Meme was coined by Richard Dawkins in his 1976 popular science bestseller, The Selfish Gene.

Public relations, advertising, and marketing professionals have embraced Internet memes as a form of viral marketing to create marketing "buzz" for their product or service. Internet memes are seen as cost-effective, and because they are a (sometimes self-conscious) fad, they are therefore used as a way to create an image of cleverness or trendiness.

Marketers, for example, use Internet memes to create interest in films that would otherwise not generate positive publicity among critics.  Used in the context of public relations, the term would be more of an advertising buzzword than a proper Internet meme.  One common form of Internet meme is created when a person, company, product, musical group, or the like is promoted on the Internet for its pop culture value.
Popular examples of use of memes in film advertising:

300 – The film 300 originated a series of image macros featuring variations of the "This is Sparta" phrase associated with images of disparate situations, often superimposing the film's main character's face onto people in the image.  This has itself since been parodied with a scene from Spongebob Squarepants featuring an enraged Patrick screaming "This is Patrick".

The Blair Witch Project – The first film to use the Internet for astroturfing. Its makers spread rumors that the material they shot was authentic and that the three protagonists really disappeared in Burkittsville.

Brokeback Mountain — inspired many online parody trailers.

Cloverfield– Paramount Pictures used a viral marketing campaign to promote this monster movie.

Mega Shark Versus Giant Octopus – The theatrical trailer released in mid-May 2009 became a viral hit, scoring over one million hits on MTV.com and another 300,000 hits on YouTube upon launch, prompting brisk pre-orders of the DVD.

Party Girl– First feature film shown in its entirety on the Internet (June 3, 1995).

Snakes on a Plane – Attracted attention a year before its planned release, and before any promotional material was released, due to the film's working title and seemingly absurd premise. Producers of the film responded to the Internet buzz by adding several scenes and dialogue imagined by the fans.