A Ninja or Shinobi, (in my mind) is the perfect warrior. A master in all forms of martial arts, (taijutsu, Karate etc) expert users of weapons such as Swords, Poles and throwing stars. Also tactically knowledged in espionage, infiltration and assasination methods. Ninjas are also masteres of there enviroment, using their surroundings to give them the best possible advantage possible i.e. using trees for infitration or hollow bamboo to breath underwater. Ninjas usually work alone and take orders without question.
Chakra is sometimes reffered to as a life force, that is contected to everyone and everything, the combination of both spiritual and physical energies (star wars all over again xd) It can be seen flowing through the body via the blood stream (using the byakugan) and sometimes visable on the outside when forced out of the body, or opening gates. Chakra is used with hand seals to create Ninjutsus.
Water or Suiton is one of the 5 basic natural elements and one of the most widely used. Waters compesition allows for it to be used in a wide varity of combinations and situations, making it very useful (Suiton: Mizu No Tatsumaki) - Water Style: Water Tornado. The abundence of Water, also allows for it to be combined with alot of other elements (Water+Earth=Wood). Water is naturally strongest against Fire and weakest against Earth.