Mobile Suit Gundam SEED is part of the Gundam anime franchise. Its universe, dubbed the "Cosmic Era" universe due to the name of its alternate calendar, is the fifth Gundam timeline to be independent of the franchise's main "Universal Century" universe. The first Gundam SEED anime began as a reimagining of the very first Mobile Suit Gundam series, but deviated approximately halfway through. With SEED's sequel, Mobile Suit Gundam SEED Destiny, the CE universe became the first (and so far only) Alternate Universe to yield more than one TV series.
Like almost every other Gundam series, the CE universe features a conflict between those living on Earth and those living in space colonies. The space faction is the Zodiac Alliance of Freedom Treaty (ZAFT), consisting of genetically-modified humans known as Coordinators; the Earth faction is the Earth Alliance, consisting of non-modified "Naturals." Relations between the two factions were strained due to ideological and economic reasons, culminating in a nuclear attack by the Alliance against the Junius Seven colony. This event marks the beginning of the Bloody Valentine War.
The first series follows Coordinator Kira Yamato and the crew of the Alliance battleship Archangel. The sequel series follows Shinn Asuka, Athrun Zala, and the ZAFT battleship Minerva during the start of the "Second Bloody Valentine War," but later shifts focus to Kira and the Three Ships Alliance.