Album Sheet and Album spread mean the same and can be used interchangeably. Whereas Album Page has a different meaning. When the Album is open, the full layout comprising of the left and the right side is referred to as Album Sheet or an Album Spread. On the other hand, each side of the album layout is knows as Album Page.
Generally while receiving orders for Album Designing, we receive instructions on number of album spreads/sheets required to design the album. Example a 30 sheet album design would mean, when the album is open the left and right side of the album layout would be considered as 1 sheet, hence 30 such sheets which is 60 pages.