What is difference between work permit and work visa?

Generally speaking, a work visa is a visa which specifically allows the visa holder to work in the United States. The most common example of this is an H visa. Persons who hold a work visa do not need a separate work permit to legally work in the U.S. The visa is the only document they need.

A work permit, technically referred to as an Employment Authorization Document, that allows a non-immigrant who holds a visa that is not a work visa to legally work in the U.S. For example, a student visa does not allow the student to work. However, a student is allowed to engage in Optional Practical Training if he or she applies for an Employment Authorization document. Other examples, in include K1 visa holders and applicant's for adjustment of status.