What does the Starcraft 2 Guest Pass do?

I have heard that Starcraft 2 contains a guest pass for a friend to use. What does the guest pass provide in terms of gameplay and time alloted?

Guest pass answer for July 2010

When StarCraft 2 was released in July 2010 this was the correct answer:

The guest pass allows a guest to play StarCraft 2 for 7 hours of game time or 14 days from activation (whichever comes first).

Once over a retail version can be purchased.

Guest pass answer for August 2011

As pointed out by marco-fiset things have changed since that time. Now the guest pass gives you access to a "Starter Edition" of Starcraft 2 which gives you the following restrictions:

• The first four missions of the StarCraft II: Wings of Liberty single-player campaign, including Mar Sara 1-3 and the choice to embark on Tychus’s first mission or Dr. Hansen’s first mission.
• The first two Challenges: Tactical Command and Covert Ops.
• Access to the terran race in Custom Games and Single-Player vs. AI.
• Access to the following custom maps (map selection may rotate over time):
  • Xel’Naga Caverns
  • Shattered Temple
  • Discord IV
  • High Orbit
• No map editor.

All campaign progress and earned achievements are saved once you purchase the full edition of the game.

The 7 hour or 14 days time restriction does not apply anymore.

ahsteele's answer used to be right, but the guest pass changed since that time. Now it gives you access to a "Starter Edition" of Starcraft 2 which gives you the following restrictions :

• The first four missions of the StarCraft II: Wings of Liberty single-player campaign, including Mar Sara 1-3 and the choice to embark on Tychus’s first mission or Dr. Hansen’s first mission.
• The first two Challenges: Tactical Command and Covert Ops.
• Access to the terran race in Custom Games and Single-Player vs. AI.
• Access to the following custom maps (map selection may rotate over time):
  • Xel’Naga Caverns
  • Shattered Temple
  • Discord IV
  • High Orbit
• No map editor.

All campaign progress and earned achievements are saved once you purchase the full edition of the game.

The 7 hour or 14 days time restriction does not apply anymore.

Source : http://us.battle.net/sc2/en/blog/3250656/

In addition to the gametime it grants a specific account, it can also authorize a local copy for the 14 days, allowing you to "Play as Guest" of the account with the guest pass.

Once registering the install with Battle.net, you can logout, and select "Play as Guest" on the main screen. You will not consume the 7 hours of playtime, however you will not receive any achievements or be able to play multiplayer.