Sirius-XM is the conglomerate of XM Satellite Radio and Sirius Radio networks after a 2008 merger was approved by the FCC.Sirius-XM is the home of exclusive radio content from the likes of Martha Stewart, Oprah Winfrey and Howard Stern.
Though they are one company, XM and Sirius still continue to operate as seperate services. This is partially because they use different technologies which are not compatable with older equipment. Subscribers to each service are offered programming from the other as a "best of" package. Some newer receivers are able to receive both services.
Sirius was founded in 1990, XM in 1992. The merged company operates 9 satellites; 5 for XM and 4 for Sirius.
Sirius-XM content is also available, with subscription, on the web as well as via iPhone and other smartphone apps.
Read HomeTheaterReview.com's review of the Sirius-XM iPhone app.
For an in-depth look at the history of the merger of the two companies, check out the Wikipedia page.