A polish is a product that will clean and polish the paint. It will take out swirl marks, light scratches, and other paint imperfections according to level of abrasiveness.
The word polish has different meanings that will change when asked to different representatives of different companies. Meguiar's is a big offender of this. They have good products, but most autopian's have more knowledge of paint care than some of the people at Meguiars.
Zaino is a polymer sealant.
Zaino will protect the paint better than a wax (more durable), but if the wax is applied often, it will protect the paint.
It comes down to what look you prefer (carnauba or sealant), how much time you are willing to spend on your car, and how often.
Jason