What is different between Tour operators and Tour Agent?

Tour Operators plan, arrange and execute tours or packages, and advertise it.

Travel agencies (Tour agencies) act as middle man between the public and the tour operators.

They make bookings with the tours operators on behalf of the public, and collect payment and give vouchers. Then they deduct commission from the payment and pay the tour operator.

They are literally "agencies" i.e they don't do any of the work on the tours or packages themselves, they only book it, and get paid by both the traveler and the tour operator do perform this service.