Once upon a time in 2001...

at Snowbird, Utah, 17 smart and intelligent “fellows” who were uncovering better ways of work, gather to talk, ski, relax and find a common ground to the need for an alternative to heavyweight software development processes…At the end the Agile Manifesto was born.

TAKE A PEEK INSIDE

Agile is...

#dealwithchange
#adapt
#adjust
#deliveryvalue
#transform
#collaborate
#reducewaste
#respect
#mywayoflife

Agile (adj.)
1. quick and well-coordinated in movement; nimble.
2. active; lively.
3. marked by an ability to think quickly; mentally acute or aware.

Described by 4 VALUES
Defined by 12 PRINCIPLES
Manifested through an UNLIMITED NUMBER OF PRACTICES

How "Agile" emerge linked to software development

In 2001, the word AGILE was the chosen one to represent better ways of work in software development. 
At Snowbird, Utah, 17 smart and intelligent people, all linked to software development, gather to find a common ground for the need to an alternative to heavyweight software development processes. After ski, relax, eat and talking a lot, the Agile Manifesto emerged composed by 4 Values and 12 Principles. This values and this principles are the base for an Agile Way of Life.

Values

There is 4:
Individuals and interactions over processes and tools; Working software over comprehensive documentation; Customer collaboration over contract negotiation; and Responding to change over following a plan.

The bold ones are more value.

Principles

1. Our highest priority is to satisfy the customer through early and continuous delivery of valuable software.
2. Welcome changing requirements, even late in development. Agile processes harness change for the customer’s competitive advantage.
3. Deliver working software frequently, from a couple of weeks to a couple of months, with a preference to the shorter timescale.
4. Business people and developers must work together daily throughout the project.
5 . Build projects around motivated individuals. Give them the environment and support they need, and trust them to get the job done.
6. The most efficient and effective method of conveying information to and within a development team is face-to-face conversation.
7. Working software is the primary measure of progress.
8. Agile processes promote sustainable development. The sponsors, developers, and users should be able to maintain a constant pace indefinitely.
9. Continuous attention to technical excellence and good design enhances agility.
10 . Simplicity–the art of maximizing the amount of work not done–is essential.
11. The best architectures, requirements, and designs emerge from self-organizing teams.
12. At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly.

In a simple way...

With 4 Values, 12 Principles, small, motivated and cross-functional teams, who inspect themselves frequently for continuous improvement. 
In short cycles delivery value to Customers. 
The Customers start to use the product, receive feedback, learn and adapt.

And the cycle resumes…