Chakra is a mix of your biological/spiritual the energy.
All jutsus require chakra. The ratio of biological to spiritual creates different effects (Like elemental things)
Elemental chakra is using chakra generate an elemental effect. Some people have affinities for certain elements. It seems to be genetic. (Clans have similar affinities) However, this doesn't mean that you can't use a elemental technique you do not have an affinity for- it just means it will not be as effective or as easy for you to train.
Demons are formed out of immense gatherings of malicious chakra that has been given a concision and somewhat a body. A host can only make use of a demon's chakra if it is sealed in him an a very powerful seal is placed upon them. It is always harmful to the host. Most of the time the demon can chose to not provide any chakra depending on how spiteful they are. (And the VAST majority are)
Nature or "Sage" Chakra is just like normal chakra except it is drawn from the world around the user and internally balanced with the other two parts. (So instead of just mind and body, the somewhat elusive "Nature" chakra is added to the stormy mix) Only a rare few have an ability to see natural chakra without aid. They are normally called "Sages" or "Hermits".
