Yesterday, I decided to restart playing its PC version for some reason. Finished the Yukine and Fuko arcs yesterday and Ryou/Kyou a few moments ago. Aiming for 100% completion.
Clannad, the anime, is a rendition of the PC visual novel game released in 2003 that primarily follows the main heroine Nagisa Furukawa, but also blends in the many other characters' arcs and plots quite well. It begins with the protagonist Tomoya Okazaki, his circumstances, and his daily life at school. From there you branch off into a wonderful story reinforcing the values of family, friends, and relationships.
Clannad After Story picks up after your school life events from the original Clannad series. It focuses on a developing relationship with the main heroine. Additionally it completes any loose ends from the first season, making this sequel, in my personal opinion, much better than the original. It is a must-watch if you intend to start this series.
For those who have seen the series, many may agree and recommend to just watch it. Searching for information about the show can be numerous and filled with spoilers, which you should avoid at all costs due to its very well-thought-out and detailed storyline that is rare to find in today's anime. It's a very popular series with highly-developed characters, worthy of your time to watch and your money should you buy the discs (and you should, in Blu-ray might I add). I must note that this is one of the very few anime that has brought a tear to my eye while watching it. Only embark on this journey if you enjoy a good story with a heartfelt ending.
Edit: Watch it subbed first.