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(click or scroll down)
http://www.law.cornell.edu/constitution/constitution.table.html#articlei
Read Article I, sections 8 and 9. Keep in mind that Congress is the legislative body -- they make laws. The judicial branch of our governments (fed, state, municipal) are the bodies by which people may (or may not) be sent to prison.
Scroll down and read Article III for the judiciary. See also Section IV on the States.