The ATSC (short for Advanced Television Systems Committee) is the Terrestrial Transmission Standard used in the U.S. and some parts of southern/central America. Transmits MPEG2 Streams but uses a slightly different modulation scheme called 8VSB instead of COFDM like DVB-T.
It has been kind of a holy war whether DVB-T or ATSC is technically more advanced. For an analysis see e.g. http://www.atsc.org/news_information/papers/1999_vsb_cofdm_comparison/ICCE99paper.PDF. In In 2001, a technical report (http://web-star.com/hdtv/mstvtestsum.html) compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB.
For a technical introduction to U.S. Digital TV systems see e.g. http://www.coolstf.com/mpeg/index.html.