Previously you could download the StarCraft 2 Starter Edition and play a limited amount of the game. With only a few campaign missions and a restriction to the Terran race, a lot of the game stayed locked away in the main game. Players with the Starter Edition could play with friends but options remained limited.
With the new "Spawning" initiative, players who download the game now access the full versions of the game when playing with friends.
If your friend who owns the Heart of the Swarm expansion invites you to a game, you will both play with all of the expansions features, maps and races. Likewise, if your friend only owns the Wings of Liberty base game, then the options you can access will stay restricted to the base game.
So as long as one of your friends owns StarCraft 2, then you can both play custom games, arcade games, and even 2v2, 3v3, or 4v4 ladder matches.
If interested, you just need to download the StarCraft 2 Starter Edition then get a friend to invite you to thier session of StarCraft 2.
If you decide to purchase the game in full, all of your ladder rankings and progress will remain intact.
To celebrate the Spawning feature release, Blizzard released a new Achievement and Portrait which will unlock when you spawn a friend into a higher version of StarCraft 2 or even spawn yourself in.
Source: Blizzard
