What is the variational principle and how does it apply to physics?

[War-Saw]

I need someone who can briefly (in easy way) explain me the variational principle or tell me where (in checked source) I can find this. I will be very greatful.

 

[lightgrav]

There are several variational principles ...
a) wave propagation (least time, a special case of Huygens wave fronts;
... generalized to Feynman path)
b) classsical mechanics , usually Hamilton's extreme action integral.