The Circle of Magic quartet by Tamora Pierce is a tale of four young mages who are brought together from drastically different walks of life in order to learn how to control their magic and find their place in the world. Each book in the quartet focuses on one of the main characters: Sandry, Tris, Daja, and Briar.
Each of them is brought from a disastrous life to live at the Winding Circle Temple in Emelan. They take up residence at Discipline Cottage, run by two Great Mages, Lark and Rosethorn. Although they initially keep to themselves and even hate the others, eventually they grow as close as family as they begin to realize their power and work together to save themselves, each other, and the only home that has ever accepted them.
Sandry's Book is the first book in the quartet, and it introduces the four youngsters and brings them together through force, desperation, and natural disaster. In each of the following books, Their relationships are expanded and strengthened as they grow and develop (both as young people and as mages).
Pierce's characters are what drive these books. Her plots are straightforward and simple but not boringly so - there is just enough plot to keep the story moving, while allowing the characters to grow and fill the story. The lessons in the books, including tolerance, crossing barriers, overcoming stereotypes, and dealing with death and destruction, are all very important and presented in a way that is very appropriate for young adult audiences. The characters are all very believable - the four are not "larger than life" or bizarrely better than any other young person in the world. They act, speak, and think like other teenagers, and they make decisions based on their own experiences and not based on some powerful grasp of "the greater good." They make mistakes, they get frustrated, and they learn how to be better people and better mages because of those experiences.
The follow-up to this quartet is a quartet called The Circle Opens. In these novels, each of the four young mages leaves the Winding Circle Temple with their teacher to visit other cities in Emelan. This quartet takes place a few years after Briar's Book (the last in The Circle of Magic quartet).
The last book featuring these characters is a stand-alone novel called The Will of the Empress, which brings the mages together again. It is set a few years after Shatterglass (the final book in The Circle Opens quartet).