Oricon is a Japanese company famous for its music ranking charts and various celebrity related surveys. Its sales charts are considered authoritative by and widely used by the J-pop industry and media as reference.
The Oricon sales charts are compiled using data from tens of thousands of retail outlets, including for music albums, singles, DVDs and video games etc. The most popular rankings are arguably for the music albums and singles.
The weekly results of the music charts are published every Tuesday on their Oricon Style [Jp] website. The above sample image is a screenshot of the weekly single chart on April 12, 2011.