From Legacy of the Confederation Wiki

Official Definition: LotC (Legacy of the Confederation) consists of single-player trigger driven maps intended for use in Blizzard Entertainment's StarCraft game series and which are created using Blizzard Entertainment's proprietary StarCraft map editing tools.

Unofficial Definition: StarCraft campaigns made by players...for players.

[edit] Quick Summary

LotC consists of fully voiced campaigns for StarCraft which began in 1999. LotC spreads its storyline over 28 individual maps and 3 campaigns for StarCraft. The LotC project team, NexusCore, intends to create an additional campaign for StarCraft 2.

Legacy of the Confederation blazed its own path in the StarCraft community with the advent of a storyline which diverged from Blizzard's. While many elements from Blizzard's storyline remained intact (such as the Overmind's invasion of Aiur, Kerrigan the Queen of Blades, the rise of the Terran Dominion, and the Kel-Morian Combine's existence), much was changed as well.

There was no United Earth Directorate (UED). Instead, a rogue cerebrate brought his Zerg brood to earth centuries before the events depicted in the original StarCraft. Without advanced technology at their disposal, earth of the time was crushed. After destroying the cerebrate with aid from the Protoss, earth rose from its ashes and formed the Confederation.

Centuries later, after earth had settled many outlying colonies, the Zerg attacked one of them. Vowing not to allow the Zerg to threaten humanity again, the Confederation sent 3/4 of its fleet, thousands of ships, on a mission titled "Operation: Astal Purge" to exterminate the Zerg and extirpate the threat permanently. After eight years of warfare and after purging 67 planets completely of all Zerg presence, the Confederation stumbled upon the Koprulu Sector. When ConFed found the Terran Dominion was breeding Zerg under despot Arcturus Mengsk, earth's war expanded to two fronts.

LotC set itself apart in the SC modding community with the use of extraordinarily large battle scenes, advanced trigger driven boss fights, custom splash screens, and it was one of only a few campaigns which were fully voiced beginning to end. NexusCore also released two custom pre-rendered FMV's to further the storyline. The deviation from Blizzard's storyline earned mixed reviews from the StarCraft community.