Mystery Booster is a supplemental set with 1694 cards, 121 foil cards, and 121 playtest cards. There are two editions. Convention edition was released first and was mainly used in limited events at conventions and unavailable at retail. The normal edition was released later. The convention edition includes the playtest cards but not the foils. The normal edition contains foil cards but not playtest cards. Probably the set is only printed in the USA.
Playtest cards are not actually playtest cards (although some are based on real playtest cards). They are merely cards that are not legal in any sanctioned format that are printed to look like playtest stickers.
Mystery Booster was sold in 15 card booster packs. Booster boxes have 24 packs.
Pack Anatomy
Packs are front facing and contain 15 cards. Each card is taken from a different list of 121 cards. Convention edition packs start with the playtest card. Normal edition packs end with a foil card. The other 14 cards are in the same order in both editions. First there are two lists of white cards (A and B), the two blue, two black, two red, and two green. Then, there is a multicolor card, a colorless card, an old frame card, and then a rare.
I observed an unusually high number of packs with an extra card. (The rate was still very low in absolute terms, but much higher than other sets.) In these cases, the would be two consecutive cards from one of the lists. (There were also a few cases where there were two cards from one list, but zero from another, resulting in packs that still had 15 cards.)