They are one and the same thing. The digital TV tuner AKA converter is a physically separate 'box', which wires into the TV AV connector, and has antenna & power connections, as well as some sort of infrared remote control capability.

The marketing people in the USA have "dumbed-down" the name of the box, from "digital TV tuner" (which is what it is) to "converter" (which means nothing!)

Your radio tuner is also a "converter" (converts radio waves to sound)

Your TV is also a "converter" (converts TV signals to picture and sound)

And your BMW engine is also a "converter" (converts fossil fuel to power to propel the vehicle)