It's wonderful when you discover that there's a single word for something that, until that moment, you could only describe with many words. The word crystallizes it, makes it singular and whole, and gives the feeling or phenomenon some permanence.
Sarah Brown introduced the blogosphere to the term and defined it thus:
...the spirit of bershon is pretty much how you feel when you’re 13 and your parents make you wear a Christmas sweatshirt and then pose for a family picture, and you could not possibly summon one more ounce of disgust, but you’re also way too cool to really even DEAL with it, so you just make this face like you smelled something bad and sort of roll your eyes and seethe in a put-out manner. Kelly Taylor from Beverly Hills, 90210 is the patron saint of bershon, as her face, like most other teenagers’, was permanently frozen in this expression.
A beautiful description, but if you're still unclear on the concept, the Flickr group I'm so Bershon will more than clear things up for you.