There are many definations about evolution.
Some biologists say Evolution is the process that brings all life on earth.And evolutionists define it as change in alleles' frequency.What Darwin said was "Descence with modification."
All are true.
To make clear about how evolution occurs, first, it is start with mutation. When our genes(DNA or RNA) replicate, some mistakes occurs, and thus the replicants vary. Remember that even the genes of identical twins are not the exact copies. And there always a struggle for survival. In that struggle, variants with the characters of more reproductive sucess will be favored for survival and unfavored ones will be eliminated. Then the variants which have more reproductive success will become dominant within species.This is what Darwin defined about evolution.
But i think it is(waht i wrote) not as clear as it should be. If u really want to know about it in detil, visit
www.evolvingcode.net and enter the classroom. There r lectures about evolution. Or read Darwin's Origin of Species, Dawkins' "Blind watch maker" and "the selfish Gene". they r really cool.
moon