I've got a new Panasonic tv that only has digital audio out, I've got an older onkyo av receiver that had multiple input types.

One such input is optical. Is the digital audio the same as optical or are they different things, the socket looks similar?

If it's not the same then I guess I need a digital audio to 'other input' converter lead. With multiple types available what's best to use ; optical, component, phono, digital coaxial? Whatever input I choose I can assign the function on front buttons so I can use either.

If the plug is the same it will work. Digital audio out (Technically S/PDIF - Sony/Phillips Digital Interface), can use an optical cable (Toslink - Toshiba Link). or a coaxial cable. You can easily convert either way using a cheap converter.

You don't have to worry about what digital audio the cable has to carry that's down to the source and destination, the cable simply transfers zeros and ones.

Your kit clearly uses toslink optical cables so simply plug it in.

They are the same thing if the sockets are the same. Most Onkyo receivers label them Digital In as well which should have given you a clue.

Also there is no such thing as a "digital audio to 'other input' converter lead". You cannot plug a digital audio output into an analogue audio input with a simple bit of wire, even more so if the digital out is optical which uses pulses of light!