Gingivitis and periodontitis are caused by dental plaque. Dental plaque consists of millions of bacteria that form a sticky glue-like substance that allows them to adhere to the teeth. If plaque is allowed to build up, the bacteria in it cause the gums to become inflamed; leading to bleeding especially on brushing.
This is called Gingivitis. If treated early, Gingivitis is a reversible condition but if it is not then it can progress to Periodontitis which is not reversible.
Where plaque has not been removed it hardens into calculus and has to be removed professionally. Occasionally, even in a very clean mouth, gums may become sore and infected, and you may experience an unpleasant metallic taste. This is acute gingivitis and requires an urgent appointment.