Established models and processes for software development try to remedy the prevailing uncoordinated chaos by adopting organisational principles from more traditional engineering disciplines. Their handicap is that they are 'heavyweight' and 'inert', essentially trying to solve problems the ordering customers had years ago, when the project was started and specifications were defined, instead of problems they will have when the software is ready for deployment.
In February of 2001, seventeen representatives of alternative, newer methods like Extreme Programming, SCRUM, DSDM, Adaptive Software Development, Crystal, Feature-Driven Development, Pragmatic Programming, and others sympathetic to the need for an alternative to documentation driven, heavyweight software development processes made up and signed the Manifesto for Agile Software Development.  The indented blocks below contain its wording, with some comments by me in between:
We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value:
Individuals and interactions over processes and tools
The idea here is that no process or tool will turn a bunch of idiots into a highly qualified team. On the other hand, qualified software engineers
will organize themselves by the principle of emergence
, when there is little management and plenty of communication and interaction. It also doesn't make much sense to write huge books about sequences of operations, responsibilities, standards, methods, tools, etc., because in software engineering, production processes don't repeat themselves and are not uniform, and trying to conduct them that way is definitely not agile
Working software over comprehensive documentation
Filling dead trees
no one's going to look at ever again is a waste of time. Instead, documentation is only created where it is necessary for a safe operation or no better forms of communication can be found.
Customer collaboration over contract negotiation
Responding to change over following a plan
Writing down a full specification of the yet nonexistant
software system at the start of the project is not possible in a sensible way. Instead, the customer should be involved in the whole creation process, ensuring the engineers produce what he needs. Plans become obsolete too fast.
That is, while there is value in the items on the right, we value the items on the left more.
Kent Beck, Mike Beedle, Arie van Bennekum, Alistair Cockburn, Ward Cunningham, Martin Fowler, James Grenning, Jim Highsmith, Andrew Hunt, Ron Jeffries, Jon Kern, Brian Marick, Robert C. Martin, Steve Mellor, Ken Schwaber, Jeff Sutherland, Dave Thomas
Since then, lots of developers signed the manifesto. They follow the following principles , which are much like Extreme Programming:
Our highest priority is to satisfy the customer through early and continuous delivery of valuable software.
Welcome changing requirements, even late in development. Agile processes harness change for the customer's competitive advantage.
Deliver working software frequently, from a couple of weeks to a couple of months, with a preference to the shorter timescale.
Business people and developers must work together daily throughout the project.
Build projects around motivated individuals. Give them the environment and support they need, and trust them to get the job done.
The most efficient and effective method of conveying information to and within a development team is face-to-face conversation.
Working software is the primary measure of progress.
Agile processes promote sustainable development. The sponsors, developers, and users should be able to maintain a constant pace indefinitely.
Continuous attention to technical excellence and good design enhances agility.
Simplicity---the art of maximizing the amount of work not done---is essential.
The best architectures, requirements, and designs emerge from self-organizing teams.
At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly.