Mutations are permanent changes in an organism's DNA sequence (which is their genetic material.) These changes can be positive or negative. Mutations that occur in reproductive cells are passed on to offspring - this is the basis of evolution and natural selection.
Mutations cause different species to develop and beneficial mutations encourage reproductive success.
Mutations are caused by many factors such as viruses, radiation, chemicals, or within the organism itself.
Examples of mutations in humans are hypertrichosis ('werewolf' syndrome where the sufferers are entirely covered in thick body hair) and Marfan syndrome (which is a disorder of tissues which causes bones to overgrow and cause joint issues).