Agile Principles

The 12 Principles of Agile Development

Customer satisfaction by early and continuous delivery of valuable software.
Welcome changing requirements, even in late development.
Deliver working software frequently (weeks rather than months)
Close, daily cooperation between business people and developers
Projects are built around motivated individuals, who should be trusted
Face-to-face conversation is the best form of communication (co-location)
Working software is the primary measure of progress
Sustainable development, able to maintain a constant pace
Continuous attention to technical excellence and good design
Simplicity—the art of maximizing the amount of work not done—is essential
Best architectures, requirements, and designs emerge from self-organizing teams
Regularly, the team reflects on how to become more effective, and adjusts accordingly

In regards to software development, you will hear "Agile Methodology" mentioned. By it's nature, Agile is a set of Principles to work to, rather than a strict methodology, so really you should talk about "Agile Principles".

Agile - able to move quickly and easily.

Methodology - a system of methods used in a particular area of study or activity.

Agile is a set of principles to work to, rather than a methodology with set methods.

Continuous improvement.

Break Work Down

When work is broken down into small, manageable chunks, it becomes much easier to visualise and prioritise.

Measure and Adapt

Measure the results of what you are doing to see what improvements you are making. It is only by measuring that you know something is having an effect. Adapt what you are doing according to whether it is working.