The analysis of population systems is carried out on the basis of the classification of spatial and functional structure of populations developed by V. N. Beklemishev. Two aspects of the structure of population systems are established. Firstly, population systems are composed of the smaller groups characterised by different self-maintenance ability. Secondly, different functional parts are included into these systems in accordance with different stages (phases) of a life cycle. Peculiarities of the population systems are discussed from these points of view. The population system is a functional part of a particular community. Steady interrelationships between population systems in the community ("community links") are the basis on which the complexes of population systems in different species are formed. A prominent example of this is the parasitare systems, that is the population system of a parasite and all connected populations of its hosts. The structure of a parasitare system is examined. In general, it is characterised by a) peculiarities of the life cycle of the parasite, since its population systems are the organising component of the parasitare system; b) subdivision of the environment for parasites. The first trait is discussed from the standpoint of phase structure of populations which is could be clearly seen in parasites, and the second one -from the viewpoint of the availability of distinct microbiotopes connected with different parts of the population system of parasites. It is the subdivision of the parasites' environment and its organisation according to the scale (interspecies, interpopulation or intrapopulation) variability of the hosts, that make it possible to recognise spatial and functional parts in the framework of the parasitare system. The critical review of the terminology used in the population parasitology is presented.