Booster packs generally contain a relatively small number of items, randomly assorted. In some games, there is a fixed distribution based on rarity, while others use truly random assortments.
Booster packs are generally priced to serve as good impulse purchases, with prices comparable to a magazine, a softcover book, a few comic books, and other such items.
Booster packs are generally the smaller, cheaper counterparts of starter packs.