The Agile Manifesto defines the Values and Principles that define Agile: Individuals and interactions over processes and tools Working software over comprehensive documentation Customer collaboration over contract negotiation Responding to change over following …

This introductory paper on Agile methods by Martin Fowler focuses on the two criteria that set Agile apart from traditional methods: adaptive planning and a people-centred approach.

This article, by Robert Martin of Object Mentor, is from the early days of XP (January 2000) and compares XP to heavyweight processes.  It also addresses the question: Does XP work?