Supernatural had a five-season story arc, with the sixth season introducing new storylines and characters. While fans were expecting that the show could continue, it has now been made official. Here is part of the official CW press release, announcing the renewal of Supernatural for a seventh season:

"The network also renewed fan favorite SUPERNATURAL, which enters its seventh season next year.   In moving to Fridays last September, SUPERNATURAL immediately helped make The CW more competitive on the night and dramatically contributed to year-to-year gains on Friday of 66% in adults 18-34 and 60% in viewers."

The sixth season is still wrapping up, with the season finale airing on May 20. The show will pick back up for the seventh season this fall.

Source: Deadline