1) In blanking, the punched out piece is useful, others are scraps. Blanking is an operation  in which a die is used to punch a kind of  shape out of a metal sheet.
The portion of the metal that has been punched out is retained, while the rest of the metal is discarded. This can be likened to cutting out a cookie with acookie cutter and keeping the cookie-shaped portion.

2) In piercing, the punched out piece is scrap. Piercing is essentially the opposite of blanking. When a sheet of metal is pierced, it is punched with a die and the punched section is discarded. This process is similar to using a paper hole-puncher where the paper is pierced and the sheet is the portion that is kept.

Observe the metal sheet  to recognize whether it has been blanked or pierced. If it has been blanked, it will be a solid shape that has been out of the sheet of metal. If the metal has been pierced, it will be a sheet of metal with a hole in it in the shape of the die. Compare blanked and pierced sheets of metal to observe the difference.