XP is an Agile methodology which takes software development to the extreme by truly embracing best practices while focusing on extreme quality and extreme responsiveness to changing customer requirements. Although XP can be used as a standalone Agile methodology for very seasoned teams, it truly brings the most value when used to expand Scrum or other Agile methodologies. Staring with very simple approaches like Scrum or Kanban to provide the right level of structure for managing the flow of the work, XP and its focus on best practices truly helps teams reach higher maturity levels.
For a gentle introduction to XP by Don Wells, make sure to visit extremeprogramming.org. According to Ron Jeffries at xprogramming.com, a leading XP practitioner, XP can be summarized as "a discipline of software development based on values of simplicity, communication, feedback, courage, and respect. It works by bringing the whole team together in the presence of simple practices, with enough feedback to enable the team to see where they are and to tune the practices to their unique situation."