The best way of describing this series is it's a semi-fantasy romantic comedy drama. It is lovely show and it'll make you cry like nobodies business.The story is about the attempt to revive the Drama Club at Tomoya and Nagisa's high school. The anime is based on the popular game visual novel.
The main female characters have their own arcs, Fuuko, Kotomi, Ryou/Kyou, Tomoyo and of course, Nagisa. Its a bitter sweet story in that since only one girl can get the guy, and they all (bar Fuuko) like Tomoya.
As the year goes by the two leading characters begin to realize that real life is more complex than any stage play as new friends and rivals make everything more complicated.A strange visitor with a unexpected message enters to change Kotomi's life forever.
The second half afterstory outlining the 20 years in the life of Nagisa and Tomoya after graduation is available on DVD.
Added 10/30/11
I bought Afterstory but discovered my edition wasn't properly licensed as well as squeezing 12 episodes onto a single layer dvd which resulted in poor picture quality. I have replaced both the Clannad Collection and the Clannad After story with the UK region 2 issues.
Clamp in Wonderland.
Various characters from the Clamp collectives manga and anime series gathered together