Baptist means you believe in actual baptism, not baby-sprinkling. They could also be considered Evangelicals, but really depends. They are usually not Calvinists or reformed. Baptists are like the most common christian in America.
Southern Baptist is just a collection of Baptist churches under the Southern Baptist Convention. All these churches share a same generic theology, but within the individual churches, there can exist different beliefs, but nothing completely different. They can be called Evangelicals.
So Baptists are the people, Southern Baptists are Baptists within a convention. Everyone in the SBC are Baptists, but not all Baptists are in the SBC.
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