Evolution is a progressive change within a species;
it happens, as I understand it, that a mutation in a gene, for example a bird has a larger beak but is one species (in this case, it would be a bird that has to dig deep to get its food), will grow better and thus reproduce more. The offspring have a chance of inherriting the same mutation, and thus, ones who have it to a more extreme extent will reproduce more and eventually will not be able to reproduce with the former species due to that their genes are too altered to do so, thus forming another species.