The largest Baptist group in America, with +/- 15.9 million members. The Southern Baptist Convention is a large group of Baptist churches that have decided to work together. They are a group of 'liberal' Baptists who believe in religious liberty, as long as you are Baptist and agree with them. There is some wriggle room; every church does not have to agree on everything (differing viewpoints on creationism and evolution are allowed, for example). On the other hand, a church cannot approve of homosexuality or abortion and still be a member.
The Southern Baptist Convention was started in 1845, in part in reaction to the failure of the Northern Baptists to move aggressively into the South, and in part because the Northern Baptists were anti-slavery (mostly the later). The SBC has since apologized for the whole slavery thing. The Southern Baptist formed a convention immediately, which met to give cohesion and centralized organization to the organization. The Northern Baptists waited until 1907 before they formed a convention of their own.
There is an annual convention, two days long, when representatives from the churches come together to speak of matters of faith. Aside from this, 42 states have state conventions, and there are over 1,200 local associations. These days the 'South' in their name is kept only out of tradition. To become a member, you must contribute to the mission causes of the church. The resulting missionary work is probably the biggest undertaking of the Southern Baptist Convention as an organization, with 4,946 missionaries in 153 different countries.
Their homepage is http://www.sbc.net/default.asp