Shinobi are the ones that endure.
They are people that have chosen and are prepared to endure whatever is thrown their way for the sake of that which they hold dear. For some that is their beloved village( Hiruzen), for some it is a person that they dear (Haku) and for some it is their dream. (Sasuke)
They are people that have chosen to defy the world and it's limitations. ( Gai & Lee) They are people who love ( Obito ) They are people who sacrifice everything for the greater good (Itachi) They are people who seek to change the world ( Nagato ) And they are people that get scared, confused, anxious... They are people. People who chose to endure, who choose to fight back. That is what a true Shinobi is.
Chakra is a mixture of a spiritual and physical energy.
It is something irreplaceable. Yes, it is the one thing that Shinobi absolutely need in order to preform any kind of Jutsu, but that is not it's true Nature. The true Nature of Chakra is that it connects people. It connects people Spiritualy and Physically, it is something that is meant to help people understand one another. It is something that has potential to unite the whole world, however, it is being abused for impure reasons. It is being used as a weapon, as something that creates an even larger gap amongst people.
I know its highly idealistic, but I figured it would be better if I wrote something by myself, instead of simply rewriting what the Wiki says U_U