ri- a prefix used when something is repeating. The verbs /tornare/ and /ritornare/ both mean /to come back/. There are contexts where /tornare/ is not enough and you must either say /tornare qui/ or /ritornare/. Example (in a shop, buying a remote control): a) E se questo telecomando (remote control) non funziona con la mia TV? b1) Lo ritorni con lo scontrino (receipt) e le daremo indietro i soldi. b2) Lo torni indietro con lo scontrino e le daremo indietro i soldi. b3) Lo torni con lo scontrino e le daremo indietro i soldi. b4) Lo torni di nuovo qui con lo scontrino e le daremo indietro i soldi. (b2) is ok, although it has too many /indietro/ in it, (b1) is better, (b3) is not good; (b4) is ok but quite long! Also there is the word /ritorno/ to mean /the action of coming back/, where ri- is just 100 % needed. I would say that we use /ritornato/ when the place of coming back is silent and inferred from the context. When the place of coming back is expressed, I would use /tornare/. Anyway if you alway use /ritornare/ you are not making a mistake, maybe some very slight redundancy. I usually try not to use redundancy in my Italian, unless for emphatic purposes. the context and

The difference is little, "tornare" means COME BACK and when you say "ritornare" means COME BACK another next time (again!)

Tornare e ritornare are synonymous. However we can use one or the other depending on the emphasis that we want to give to the phrase. E.g. if I say "sto tornando a casa" I could refer to my return to home after a work day. I can say " sto ritornando a casa" for example to underline that I come back home after a period of absence or for the umpteenth time.