What is the difference between a state legislature and congressman?

Answer 1

One is a deliberative body and the other is a human being. 

Answer 2

A state legislature in the United States is the legislative body of any of the 50 U.S. states.

A congressman is a member of a congress, especially a member of the U.S. House of Representatives 

Answer 3

None IF the congressman works in the state legislature.  There are senators (who work in a state's Senate) and representatives (who work in the state's House) in the state government and there are senators (the Senate) and representatives (House of Representatives) in the federal government.

Not all states have a state Senate and House but 41 of them do.

The difference between the state legislature and the federal legislature is that laws passed by the state and signed by the governor impact only the residents of that state.  Laws passed by the federal legislature and signed by the president impact everyone in the US.