Phenotype is influenced by genes and by the environment. Genetic variation means different characteristics caused by differences in genes. There are two main causes: mutation and sexual reproduction.
A mutation is a random change in the base sequence of DNA. Mutations can create new alleles, which are different versions of a gene. A new allele may give a new characteristic.
In sexual reproduction, offspring inherit a mixture of genes from two parents. Each gamete is genetically different from the others. At fertilisation, any male gamete can fertilise any female gamete at random. Offspring therefore differ from their parents and from each other. Asexual reproduction does not do this, because it produces clones.
Differences between identical twins cannot be caused by their genes, because their genes are the same. Such differences are caused by the environment, for example diet or exercise.