MLB.com is the official website for Major League Baseball. It includes news and information on all MLB teams and players, statistics and various sports columns about the game itself. The site also features live streaming audio and video for all MLB games under a subcription service. The site offers official merchandise for sale, and runs auctions for various types of baseball memorabilia.
One can also consult the official rules of the game on the site. The sites includes sections in Japanese, Korean and Spanish; there were also French pages in the past, although those were discontinued when the Montreal Expos moved to Washington, DC after the 2004 season.
MLB.com acts as the umbrella site for Major League Baseball on the internet. Individual teams have their own sites as subdomains under MLB.com (eg. toronto.bluejays.mlb.com). The site is run by Major League Baseball Properties Inc. in New York City, and has been available on the internet since 1994. 
The site offers fans a chance to register for a personalized log in, thus allowing them to access a webmail account for their favorite team, and to vote/participate in various contests. Fans can also discuss the sport of baseball on various forums.
The domain name was acquired by MLB in 2000 from the law firm Morgan, Lewis and Bockius LLP, which had previously used the site (forcing MLB to use www.majorleaguebaseball.com as its main domain). Ironically, the law firm had been doing legal work for Major League Baseball for a number of years, particularly in the area of labor law, and the transfer of property was done for little or no monetary consideration.
sources: MLB.com, bluejays.com, Wikipedia article on mlb.com, Whois search for mlb.com
- Ross E. Davies: "The Law Firm and the League: Morgan, Lewis and Bockius LLP, major League Baseball, and MLB.com", The Baseball Research Journal, SABR, Vol. 39, Number 2 (Fall 2010), pp. 74-80.