The difference between them is that: you use "questo" when you want to refer to something close to you, like "this" in English. Conversely you use "quello" when you want to refer to something away from you, exactly as " that" in English. I hope I've been helpful :)

The previous answers are perfect. I can only add that in italian you have to coniugate these adjectives with the feminine and masculine and with the singural or plural to which these adjectives refer to.

Dici "questo"a qualcosa che è vicino a te (come "this"in inglese). Mentre "quello" a qualcosa che è distante da te (come "that"in inglese)

Grazie a tutti @