Naruto Manga: Chapter 17-Preparation For Battle!!

"Simply put, chakra is the energy a ninja needs when performing a jutsu. That energy has two parts. The body energy that is in each of the billions of your cells. And the spiritual energy gained through training and other experiences. These two parts are combined. So by bringing out the chakra, you can use a Jutsu. These is done through the process of performing a seal with the hands." [Sakura]