Chakra is the molding of the physical energy present in every cell of the body and the spiritual energy gained from exercise and experience. It can be channeled through the chakra circulatory system, which is to chakra as the regular circulatory system is to blood, to any of the 361 tenketsus in the body.
Ninjutsu means ninja techniques, it is a term referring to almost any technique which allows the user to do something that they otherwise would be incapable of doing, including the usage of weapons. Unlike genjutsu, which makes the opponent see illusions, the effects of ninjutsu are real, and they mostly require chakra manipulation.
Elemental ninjutsu is the combination of nature transformation and shape manipulation to create/manipulate the elemant a ninja usually has affinity for. It is the molding and defining of one's chakra into an particular type of chakra nature, altering its properties and characteristics, and also manipulating it into different shapes and form.
Suiton is one of the basic elemental nature transformation techniques that allow the user to manipulate pre-existing water, or create their own, by changing their chakra into water. Alongside the shape, the state can also be changed, eg steam, ice
Water is weak to earth, as earth would easily stop its flow, it is strong to fire, as it can easily put out fire, creating steam