Agile might be called iterative and incremental development or adaptive project management.
Perhaps the next shortest explanation of Agile is the Agile Manifesto.
The first principle (that the creators of the Agile Manifesto identified) is:
Our highest priority is to satisfy the customer
through early and continuous delivery
of valuable software.
This principle implies that Agile is only about delivering software. Those of us more focused on agile project management would say that Agile is mainly about projects -- projects of any sort (delivering software, a new product or whatever).
In this first principle many will also recognize some key ideas also associated with Lean. Deliver at the pull of the customer the value as defined by the customer.
The Agile Manifesto says:
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
Working software over comprehensive documentation
Customer collaboration over contract negotiation
Responding to change over following a plan
That is, while there is value in the items on
the right, we value the items on the left more.
See agilemanifesto.org for more information on the Agile Manifesto and Agile Principles.
The Agile Principles, while not as eloquent, are still quite good.
Agile is also associated with the following:
Note: It is rather hotly debated whether the Unified Process (AUP or the Rational Unified Process (RUP)) is or can be Agile. Just be aware.
We have heard firms say they adopted Agile for the following reasons:
For more information please see Resources and ScrumResources.
See also http://en.wikipedia.org/wiki/Agile_software_development#Adaptation_of_agile_methods
for a relatively short discussion.
See also this HBR article about new product development and faster product evolution: New New Product Development Game by Takeuchi and Nonaka. This article led to the development of Scrum, and these ideas are embedded in all Agile flavors.