Just finished the official site of Gundam SEED DESTINY, which is still very barren and lacking. So here's how I understand it right now.

[Situation]
DESTINY takes place in CE 73, two years after SEED. The leader of PLANT now is Gilbert Dulandal. The leadership on the Alliance's part is still unknown. Cagalli is the leader of Orb. There is going to be some kind of secret negotiation between leaders in the series, at least between Orb and PLANT. Gilbert Dulandal is getting accompanied by Shin Asuka, and Cagalli by Athrun.

[Character]
Shin Asuka (シン・アスカ), the main character of the series, is a 16-year-old Orb-born Coordinator. He witnessed the lost of his parents and younger sister during the invasion of Orb by the Earth Alliance in CE 71; the only thing he knew was that Mobile Suit Gundam which had flew overhead was responsible. He left for PLANT and joined ZAFT, where he became a member of the Elite and the official pilot of Impulse Gundam.

Athrun Zala (アスラン・ザラ), now 18, is taking refuge in Orb after being discharged from ZAFT and using an alias. He acts as the bodyguard and support of Cagalli Yula Athha (man, I hate those idiotic official spellings).

Gilbert Dulandal (ギルバート・デュランダル) holds the highest position in PLANT's high council after the Junius Treaty (cease-fire treaty). He is a man after Siegel Clyne's heart and wishes to resolve the tension and conflict between Coordinators and Naturals. He is trusted by the people.

Lunamaria Hawk (ルナマリア・ホーク), Coordinator, is a crew member on the ZAFT battleship Minerva (also the mothership of Impulse). She is also an Elite MS pilot.

[Mech]
Impulse Gundam (インパルスガンダム), ZGMF - X56S, the latest suit developed by ZAFT and designated suit of Shin Asuka, who prevented it from being seized. Because the treaty prohibited the use of nuclear engine, it uses a new Dutolion (デュートリオン) beam system for power. This enables wireless transfer of energy, and staying within range of the mothership gets it supplied (kinda reminds me of the Aestivalis from Nadesico), making it possible for long-term phase shift. The cockpit can transform into Core Splendor (コアスプレンダー) fighter or call for silhouette systems (like Strike's backpacks) according to the tide of the battle.

Force Impulse Gundam (フォースインパルスガンダム), ZGMF - X56S/α, is basically Impulse with the Aile pack for high mobility with beam rifle and beam sabre as its main weapons. There are other types such as Sword Impulse and Blast Impulse.