Some very good questions coming form the source at Blizzard.
What is the StarCraft II Trilogy?
The StarCraft II Trilogy consists of the base StarCraft II game and two subsequent expansion sets. StarCraft II is subtitled Wings of Liberty (working title) and will include a lengthy single-player campaign that focuses on the terrans and puts players in the role of Jim Raynor, one of the series’ main heroes. The first expansion set, Heart of the Swarm (working title), will follow later and ...