Blizzard has replaced the demo for StarCraft II with an even bigger sampler. The StarCraft II Starter Edition gives players a more in-depth look at the single-player and multiplayer components of the best-selling real-time strategy.
Like the original demo, the Starter Edition lets you play the first three missions of the single-player campaign on Mar Sara. However, then you're given a choice: you can perform a mission for mercenary Tychus Findlay or the scientist Dr. Hanson. The full campaign branches a lot like that so this gives you a taste for the decisions you'll need to make.
Two Challenges, called Tactical Command and Covert Ops, are also included. Challenges are standalone missions that teach players both basic and complex techniques for playing SC2. They're a good way to prepare for the rigors of the campaign or online play.
Speaking of online, you can also join Custom Games on four maps: Xel’Naga Caverns, Shattered Temple, Discord IV, and High Orbit. You're restricted to playing the Terran race during these matches, though. You can play with other Starter Edition players or owners of the full game.
Last but not least, Starter Edition owners will get access to the map editor that ships with SC2. You won't be able to publish maps to Battle.net or use the Test Map function. However, it should give you an idea of the capabilities of the toolset.
If you decide to upgrade to the full edition, the transition will be pretty seamless. Blizzard says that all of your campaign progress and achievements from the Starter Edition will carry over.