There are 6 Common cards, 1 Common or The List card (The List being special cards, usually reprints, the selection of which can change between sets), three Uncommon cards, 1 non-foil Wild Card (this can be anything from the main set that the booster is a part of), 1 Rare or Mythic Rare from the boosters set, 1 Foil card of any rarity, 1 Basic Land card and, finally, 1 non-playable card, which can be a token, play aide, ad card or art card
Take Faithless Looting for example
If you already have Kingdom of Germa set up on the field, you can Trash another copy of it in hand for a card draw
Secret lairs are mini-sets containing a handful of cards a regular Magic expansion wouldn't have room for
Also, this sweeps graveyard too