Noughts and Crosses by Malorie Blackman is book one of a trilogy.

Persephone Hadley is a Cross, daughter of a prominent politician, while Callum McGregor is a nought. As best friends, their relationship is frowned upon by society, a secret to Sephy's mother, and not approved by Callum's family.
As they grow up their relationship is tested as their two worlds conflict.

This book is hard to sum up in a paragraph ! but i assure you that it is a FANTASTIC book