The most striking difference is described in this page
Other differences concern the use of articles. Eng. possessives are never preceded by articles, It. possessives are normally preceded by a definite article (articolo determinativo) as the option. Examples: "La mia auto è quella gialla. Oggi pomeriggio gioco con i miei amici. I miei libri e i suoi." ( My car is the yellow one. I'm playing with my friends this afternoon. My books and hers.) The definite article is omitted in only a few cases - for example, with nouns referring to relatives. Example: "Guarda! Arrivano mio padre e mia sorella." ( Look! My father and my sister are coming.)
An It. possessive can be preceded by an indefinite article (articolo indeterminativo), meaning "one of". Example: "Un mio amico ora è in Irlanda." ( One of my friends / A friend of mine is in Ireland now.)
Likewise, It. possessives can be preceded by:
demonstratives. Examples: "Questa mia cravatta è francese. Quelle vostre biciclette non devono stare lì." ( This necktie of mine is French. Those bicycles of yours cannot stay there.")
numerals. Examples: "Tre suoi amici sono calciatori. Due sue sorelle studiano a Pavia." ( Three friends of his are footballers. Two of her sisters study at Pavia.)
indefinites ( some, any, every, etc.). Examples: "Alcuni nostri paesaggi sono famosi nel mondo. Qualsiasi sua iniziativa è un insuccesso." ( Some landscapes of ours are world-famous. Any initiatives of hers are flops.)