This sequel to 'Clannad' starts shortly after the events in that series and covers the events in Tomoya Okazaki's life for the next seven years. We see his relationship with Nagisa develop and lead to marriage and the birth of Ushio, a daughter. There are various events which will have life changing effects on those involved. The story involves various alternate realities where we see the events play out in different ways.
I've tried to keep my precis as vague as possible as there are certain key events which shape later parts of the story which I'd rather not spoil. To use a cliché this series is an 'emotional rollercoaster'; at times it is very funny with both slapstick humour and character driven humour at others it is genuinely tear jerking... I don't mean 'there's something in my eye' tears but proper sobbing! This is because the characters involved are well developed and feel real. The animation is of a high quality throughout. Overall I'd say this is a must see if you enjoyed the first series; I wouldn't advise going straight to this as certain elements might seem more than a little confusing.
These comments are based on watching the series in Japanese with English subtitles.