IntERspecific - The variation that exists between different species.
IntRAspecific - The differences that occur within a species. Intraspecific variation is caused by genetic and environmental factors.
All members of the same species have the same genes, but have different alleles which make up our genotype. Different genotypes result in different phenotypes - charachteristics. You inherit these from your parent, so genetic diversity is inherited.
Environmental factors also have an affect, e.g. nutrition, fittness etc.
Variation is often a combination of both environmental and genetic factors e.g a person may have genes to grow 6ft tall but it all depends on their health and diet.