Evolution is the mutation of genetic information in the replication process as an organism grows. In asexual reproducing organisms(single cell life forms) the mutations occur in the splitting of the chromosomes(long chains of DNA which code for specific groups of traits, eg gender, facial appearance). The 'daughter' organism is supposed to be an exact replica or clone of the 'parent' but often they are not. Because the replication process has no failsafe where by an agent checks that the genetic information has been replicated accurately, and corrects the mistakes, the 'daughter' continues existing with the mistakes or mutations. These are not necessarily beneficial, and more often than not they are detrimental. The beneficial mutation is anything which provides advantage and therefore better survival skill. They then breed and pass on their advantage, though not always, the advantageous genes may be replicated incorrectly and mutated into something detrimental, it is a matter of chance.

Sexually reproducing creatures are always multi-cellular. The cells though still reproduce asexually, therefore they can come out with errors, these errors may may them incapable of performing their programmed task, to their detriment, or better at their task, benefit. There is a natural tendency for rejection of unique mutant cells, the same is true in organisms, therefore they tend to breed with each other, this forms a stronger group which competes with the mainstream and becomes dominant(the new mainstream) these traits are then attractive in reproductive pursuit and become stronger as the population becomes filled with the beneficial mutation. At some point there was a finch which had a beak which made it better at feeding due to a mistake in the gene replication process, it bred and soon all the finches with out the new 'super beak' were out competed. Really its far more detailed but I can't be bothered explaining further.