Differences between individuals of the same species are called variation. Some variation is genetic variation: different characteristics inherited in the genes. It has two main causes, mutation and sexual reproduction.
A mutation is a random change in the DNA of a gene. It can create a new allele. Most mutations have no effect on the phenotype, but a few change it, and a new allele can then be passed on to offspring.
Sexual reproduction mixes alleles. Gametes are made by meiosis, so they are genetically different from each other, and each carries a random mix of alleles. Which gamete fertilises the egg is also a matter of chance. Offspring therefore inherit alleles from two parents, and differ from both parents and from their brothers and sisters.
Other differences are caused by the environment. These are acquired characteristics, such as a scar or the effect of diet on body mass. They are not caused by a change in the genes.