Keeping it simple, it is the interaction, what makes ecology as complex as it is. Basically, the interaction between an organism and its environment. This environment can be biotic or abiotic or a mixture of both, which in all its completeness would be called an ecosystem. On the species level, it would be the interactions between individuals within the same species or an individual of one species with its environment, called intraspecific or species specific. Interspecific interactions take place between different species.
Taking all individuals of one species in a given place together, it would be called the population level. Considering also the interactions on species level, the population level includes the interactions between populations of one species or two or more species as well as their interactions with their environment. The community level would integrate the interactions of several species, living together in a spatio-temporal structure.
Ecology tries to find patterns in all these interactions, answering questions about e.g. demography, population and community dynamics, migration, isolation, predator-prey relationships, competition, regulation or adaptation.