The following is the original Agile Manifesto (http://agilemanifesto.org/):
"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."
It's pretty simple but very powerful if you are able to apply it. It can also be easily mistaken and used as an excuse to avoid doing any management. Let's look at each of the values.