Study of allele frequencies and how they change over time.
Population genetics is the means by which evolution is studied at the level, not surprisingly, of populations and genetics. In particular, this discipline is not so much concerned with the physical nature of adaptations, or how mutations physiologically or morphologically give rise to changes in phenotype.
Population genetics instead addresses issues that are more abstract, such as under which circumstances an allele might rise in frequency towards fixation, fall towards extinction, or other otherwise persist within a balanced polymorphism. The principle phenotype considered by population geneticists thus is Darwinian fitness.
Other issues of importance are the extent of random mating (panmixis), the impact of clonality on adaptation, frequencies of mutation types (beneficial, neutral, or detrimental), the impact of combining alleles, speciation, balanced mutation, etc. These are all in addition to the standard evolutionary processes of mutation (including mutation rate), genetic drift, migration, and, of course, natural selection.
See also population ecology and population biology as well as Hardy-Weinberg equilibrium.