StarCraft: Brood War is an add-on for StarCraft (requiring the full version of StarCraft) and a direct continuation of its story.
With the demise of the Overmind on the Protoss home world at the end of the original game, the Zerg are left leaderless. In an attempt to regain control, a number of cerebrates merge to form a new Overmind. Kerrigan, a human/Zerg hybrid however wants to take control of the Zerg herself. The Protoss are evacuating what is left of their people from their ruined home world. Meanwhile the United Earth Directorate (UED) have learned of the recent developments in the sector. Fearing for the safety of Earth they sent an expeditionary fleet there to secure it.
Brood War brings several changes to the game, altering some gameplay aspects of the game, the most important change being the addition of new units.
The Terrans receive the Valkyrie, a craft with air-to-air fighting capabilities. They also obtain the Medic, which can heal all non-mechanic units and, through upgrades, can use the Restore (remove all ailments on a unit) and Optical Flare (reduce the sight range of an enemy unit to 1) powers.
The Zergs receive the Lurker, obtained after mutating an Hydralisk, which can, and actually must, burrow to attack. They also gain the Devourer, after mutation of a Mutalisk, an air-to-air unit, whose acid shots have notably the specificity of temporarily lowering the enemy defense, allowing other units to do increased damage.
The Protoss receive the Corsair, an air-to-air unit, which has access to the Disruption Web power (which, once cast, prevents targeted enemy units from firing outside of the web). The Dark Templars, already seen in the original StarCraft are now a producable unit. Their main strength lies in their permanent cloaking, allowing them to deliver their strong close combat attacks unharmed. Additionally, in the same manner as two Templars can be merged as an Archon in StarCraft, two Dark Templars can now be merged to obtain a Dark Archon. A Dark Archon has access to several powerful powers: Mind Control (giving permanent control of the targeted unit), Maelstrom (temporarily rendering all non-mechanical units within the target area unable to do anything) and Feedback (removes from a target unit the amount of energy it has available for special powers and inflicts it this amount as physical damage).
Besides the new units, some units already featured in StarCraft can now be upgraded in certain areas. For instance, the Terran Goliath can have the range of its ground-to-air missiles be tremendously increased. Similarly, the Zerg Ultralisk can now receive speed and armor upgrades.
Other changes include a better AI and some alterations of the cost of certain units.
Three new campaigns are available. In order, a Protoss campaign featuring 8 missions, a Terran campaign also featuring 8 missions and a Zerg campaign comprising 10 missions (plus a bonus mission if some requirements are met in mission 9).
The add-on also comes with about 100 multiplayer maps.