Clannad's story revolves around Tomoya Okazaki, a third year high school student who dislikes his life. Tomoya's mother died when Tomoya was young, leaving his father to raise him. After the accident, Tomoya's father turned to alcohol and gambling. Tomoya meets by chance Nagisa Furukawa, a soft-spoken girl . Her goal is to join the drama club, but they find that it was disbanded. Since Tomoya has a lot of time to kill, he starts to help Nagisa in reforming the drama club. During this period, Tomoya meets and hangs out with several other girls who he gets to know well and help with their individual problems.
Clannad is the saddest Anime you'll ever watch. After every episode you ignore the fact that you get mentally fucked up (because of the sadness, really) and continue watching. A very addictive story that won't ever let you go. And the most beautiful in its own way.
Clannad does not have a real plot. First you think it's about Tomoya helping Nagisa to reform the drama club, but that's more secondary. More important are the girls he meets in that time. You get to know their pasts and problems and how Tomoya helps them to solve it. You can get ready to cry after a few episodes (it starts beautiful and funny, but then...).
Clannad After Story is kinda the 2nd season of Clannad. It's about Tomoyas life after school. New things happen and you'll never guess how it goes on! Clannad After Story is even (MUCH) better than the first season. It's more beautiful and sad at the same time. I cried in nearly every episode. More short stories about some characters, plus a really good plot! Yeah, in contrast to the first season we have a plot!
If there's an anime you can expect EVERYTHING from - it must be Clannad! It's full if surprises, beautiful characters and above all: very strong feelings. You'll never regret watching this.