Its a government (Office of Management and Budget) definition of region with strong economic ties. Its basically an urban core and surrounding communities (counties) that are linked to the core through commuting. The minimum standard, I believe, is 25% of the working population in a county must commute to the MSA for work to be included in that MSA.
Also as Andrea mentioned, Dallas and Houston are larger than Atlanta in land area with just 10-12 metro counties because of Georgia's small counties.
About MSA's