In this book Dawkins describes the concept of "selfish gene" as the major driving force of Darwinian evolution. The book concentrates on gene-centric view of natural selection.
A startling effect of selfish genes accumulating in a gene-pool (the whole set of genes in breeding population) seems essentially the reduction of individual's role in evolution to being a mere "survival machine" for increasing gene numbers and their longevity in the gene-pool.
This basic idea then is progressively applied to different areas starting from genes against alleles, individuals against other individuals, battle of sexes, individuals against changing environment, etc. in order to explain natural selection process from genes' point of view.