Neutrophils are the most common type of white blood cell. They destroy bacteria (and other parasites like fungi) during an infection. They get to the site of an injury within minutes. They have tiny granules full of enzymes which dissolve the bacteria after they have been taken in.
The process of taking in bacteria is called phagocytosis. The bacteria end up in a vacuole inside the neutrophil. The enzymes then get to work on the bacteria. Neutrophils are an important part of the body's immune system. They are a kind of granulocyte, and have a short life-span of a couple of days.