the road so far
Supernatural is a TV show from the states that follows two brothers, Sam and Dean Winchester, as they road trip across America in a '67 Chevy Impala fighting any manner of evil that they come across. And there is a lot of evil out there, from ghosts and vampires to tricksters and Bloody Mary. Originally it was conceptualized as a show where it's a different horror movie every week, but over time it started to focus more on the relationship between Sam and Dean and the importance of family (while keeping the copious amounts of terror and blood).
To make a very long story short, at the end of the second season Sam died and Dean made a pact with a demon to trade his life for Sam's. This was good in that Sam wasn't dead anymore, but bad in that by the end of season three Dean was dragged into Hell - a lovely place not unlike the movie Hellraiser. While down there, one thing lead to another and somehow the Apocalypse was put in motion, leading the armies of Heaven to pull Dean out of Hell and throw him straight into the front line on Earth. So our intrepid heroes Sam and Dean find themselves spending season four trying to keep the end of the world from kicking it into full gear.
And Then Came Anna
Without going into too much detail, at first Anna appeared to be a normal human girl - albeit a patient in a mental institution, but it's eventually discovered that she is in fact a fallen angel. This complicated her life greatly, as it made her a liability to the armies of Heaven and a tasty treat for the armies of Hell. Thank God there's always the Winchesters there to save the day! For the full story, may I suggest you start off here?
Also, for those who may be interested, the actress who plays Anna is Julie McNiven. She hasn't had many other roles of note, but she does play the small part of Hildy on Mad Men.