Warning: wikipedia contains spoilers
The book is written in letter form, the central character being Celie, a young woman who is sexually abused by her father (who, she later discovers, is her stepfather) and is forced to marry a widower with several children, who is physically abusive towards her.
When her husband's mistress, singer "Shug" Avery, comes on the scene, life changes for Celie and she learns how to stand up for herself. The women go away together and start a successful business. Celie is reunited with the sister she believed dead.
This book is often argued to address many issues which are important to understanding African-American life during the early-mid 20th Century. Its main theme is the position of the black woman in society, as the lowest of the low, put upon both because of her gender and her color.