These are guest passes:
I had 3 of them in the retail boxed version of the game. Each one has a one-time-use code, which allows another person to download the game, and play it with certain restrictions.
On the back, it reads:
The guest account allows you to play Diablo 3 for free up to the Skeleton King boss in Act 1. Note that certain game features are restricted on guest accounts - to remove these restrictions, please upgrade to the full version of the game.
Blizzard's FAQ on this system says that the Guest Pass grants you the "Starter Edition" of the game, and says the following about restrictions:
- Act I up to the Skeleton King is available
- Level 13 cap
- Matchmaking available only with other Starter Edition players
- No Real Money Auction House access
- Global Play is not available. Players attempting to connect to Diablo III Starter Edition in a region other than their Battle.net Account's home region will receive Error 12. See the Global Play support article for more information.
No Guest Passes are included with the downloadable version that you can purchase directly from Blizzard.
Some related Guest Pass questions:
Do guest pass characters carry through an upgrade to the full game?
Does a Diablo 3 guest pass let you play with regular players?