Look at any population of one species, such as the sparrows in a park or the daffodils on a verge, and you will see that the individuals are not identical. There is usually extensive genetic variation within a population. This means that individuals carry different alleles of many genes, so they have different genotypes and often different phenotypes.
All of this variation arises through mutations. A mutation is a random change in the sequence of bases in DNA. A mutation can create a new allele of a gene that did not exist before. As organisms reproduce, new alleles spread through the population, and sexual reproduction shuffles the alleles into new combinations in each offspring.
Mutations happen by chance, for example when DNA is copied before a cell divides. The rate can be raised by ionising radiation and by some chemicals. Because a species has so many genes and so many individuals, a population builds up a large store of different alleles.