I'm going to try to tell u what Gundam SEED is about without ruining anything so i'm not going to go too much into it.

Basically on the first DVD you're introduced to the main character Kira Yamato.  Kira is a coordinator and ends up being forced to fight when the Zaft organization attacks.  Zaft is basically the army of Coordinators that's fighting against the Naturals.  The Earth Alliance is the army of Naturals.  Kira being a Coordinator is considered as traitor to his fellow Coordinators.

During his first battle he meets up with his old best friend Athrun Zala.  They had been best friends since they were kids and now they found themselves fighting on opposite sides, fighting as enemies.

That's pretty much the gist of the story.  But there's a lot more to it and if u want to know more about it you'll just have to watch it for yourself.  I will say that it is by far the best Gundam series i have ever seen.  I highly recommend watching it.

Miriallia Haww~ She's a Natural and a close friend of Kira's.  She helps out on the ArchAngel as a CIC.

Flay Allster~ She's a Natural who hates all Coordinators except for Kira.  In my opinion she's a little insane.  She's the daughter of an Earth's Forces Alliance Official.  She becomes very depressed and insane (again this is just my opinion) after her Father is killed in an explosion right in front of her.

Murrue Ramius~ The Captain of the Earth's Forces Ship the ArchAngel.  After being rescued by Kira she then forced him and his friends to stay with her until she figured out what to do with them since they had seen some of the military's secrets. 

Natarle Badgiruel~ One of the surviving officers aboard the ArchAngel.  She helped Captain Ramius run things.

Mu la Flaga~ An excellent soldier with amazing fighting skills who helps to protect the ArchAngel.

Kazui Bushirk~ A close friend of Kira's who doesn't really play much of a role in the show.  He does help with the ArchAngel.

Tolle Koenig~  Another close friend of Kira's and Miriallia's boyfriend.  He helps on the ArchAngel and flies a Sky Grasper in battle when needed.  He is unfortunately killed in battle.

Sai Argyle~ A good friend of Kira's who was Flay's boyfriend until she dumped him for Kira.  He helps on the ArchAngel as well.

Dearka Elthman~ Pilot of the stolen Buster Gundam.  A friend of Athrun.

Yzak Jule~ A hotheaded pilot of the Duel Gundam.  He dislikes Athrun and is friends with Dearka.  After being beaten by Kira he decides to be the one to defeat Kira.

Nicol Amarfi~ He is a good friend of Athrun's who pilots the Blitz Gundam. He likes to play the piano.  He also loses his life in battle.

Lacus Clyne~ Athrun's fiance that falls in love with his best friend Kira.  She is a singer.  She is extremely nice.  A little too nice for her own good but ya. She helps Kira and Athrun both decide what it is that they were fighting for.  She convinces Athrun to go talk to Kira and they end up fighting on the same side together.

Andrew Bartfeldt~  He is known as the Desert Tiger.  He is a Zaft commander who takes a liking to Kira.  Him and Kira fought a tough battle that would only end in one of their deaths.

Patrick Zala~ Athrun's father who becomes the leader of Zaft.  He believes that the war will not end until every Natural is dead.

Cagalli Yula Atha~ The daughter of Uzumi Atha.  She was saved by Kira in the first DVD as is reunited with him.  She begins to take a liking to him.  After being shot down she meets Athrun and immediately dislikes him because he is a Zaft pilot and she hates Coordinators.  But she later takes a liking to him.

Uzumi Atha~ Cagalli's father and a very important representative of Orb (the Neutral nation).