Between 1997 June and 2001 February the Two Micron All Sky Survey (2MASS) collected 25.4 Tbytes of raw imaging data covering 99.998% of the celestial sphere in the near-infrared J(1.25{mu}m), H(1.65{mu}m), and Ks(2.16{mu}m) bandpasses. Observations were conducted from two dedicated 1.3 m diameter telescopes located at Mount Hopkins, Arizona, and Cerro Tololo, Chile. The 2MASS All-Sky Data Release includes the FITS images covering the entire sky, a Point Source Catalog (PSC) of 471 million sources (Cat. II/242), and the present Extended Source Catalog. The 2MASS Extended Source Catalog contains sources that are extended with respect to the instantaneous PSF, such as galaxies and Galactic nebulae. The algorithms used to create the 2MASX catalog are described by Jarett et al. (2000AJ....119.2498J), and in the 2MASS Explanatory Supplement (accessible from the 2MASS Home Page). Briefly, point/ extended-source discrimination was conducted for each band-merged point-source detection by comparing a variety of radial shape, surface brightness, image moments, and symmetry parameters with characteristic stellar parameters using an oblique decision tree classifier. The classification tests included filters to exclude double and triple stars, which were one of the main contaminants in high source density regions. Stellar parameters were measured empirically as a function of time in each scan to compensate for variations in the atmospheric seeing using the aggregate properties of band-merged point-source extractions. The catalog contains 389 columns described briefly in the "Byte-by-byte Description" section below; their description includes also the 2MASS database original column names used in the original descriptions. Acknowledging 2MASS in Publications: Please include the following standard acknowledgement in any published material that makes use of the 2MASS data products: "This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation."