Here’s a more in-depth–though, still brief–article on the differences between USL and USO.  Apparently, contango is the term used to describe the strategy of rolling over current month’s futures to the next month’s futures.

http://www.marketrap.com/article/view_article/9166/watch-out-for-contango-looking-for-the-best-long-term-oil-fund-uso-oil-usl-or-dbo