Agency is the capacity of an actor to act in a given environment. The capacity to act does not at first imply a specific moral dimension to the ability to make the choice to act, and moral agency is therefore a distinct concept. In sociology, an agent is an individual engaging with the social structure. Notably, though, the primacy of social structure vs. individual capacity with regard to persons' actions is debated within sociology. This debate concerns, at least partly, the level of reflexivity an agent may possess.
Agency may either be classified as unconscious, involuntary behavior, or purposeful, goal directed activity (intentional action). An agent typically has some sort of immediate awareness of their physical activity and the goals that the activity is aimed at realizing. In 'goal directed action' an agent implements a kind of direct control or guidance over their own behavior.
Agency is contrasted to objects reacting to natural forces involving only unthinking deterministic processes. In this respect, agency is subtly distinct from the concept of free will, the philosophical doctrine that our choices are not the product of causal chains, but are significantly free or undetermined. Human agency entails the claim that humans do in fact make decisions and enact them on the world. How humans come to make decisions, by free choice or other processes, is another issue.
The capacity of a human to act as an agent is personal to that human, though considerations of the outcomes flowing from particular acts of human agency for us and others can then be thought to invest a moral component into a given situation wherein an agent has acted, and thus to involve moral agency. If a situation is the consequence of human decision making, persons may be under a duty to apply value judgments to the consequences of their decisions, and held to be responsible for those decisions. Human agency entitles the observer to ask should this have occurred? in a way that would be nonsensical in circumstances lacking human decisions-makers, for example, the impact of comet Shoemaker-Levy on Jupiter.
The philosophical discipline in charge of studying agency is action theory. In certain philosophical traditions (particularly those established by Hegel and Marx), human agency is a collective, historical dynamic, rather than a function arising out of individual behavior. Hegel's Geist and Marx's universal class are idealist and materialist expressions of this idea of humans treated as social beings, organized to act in concert. Also look at the debate, philosophically derived in part from the works of Hume, between determinism and indeterminacy.
Structure and agency forms an enduring core debate in sociology. Essentially the same as in the Marxist conception, "agency" refers to the capacity of individuals to act independently and to make their own free choices, based on their will, whereas "structure" refers to those factors (such as social class, but also religion, gender, ethnicity, subculture, etc.) that seem to limit or influence the opportunities that individuals have.
Other notions of agency have arisen in the field of economics/management, psychology and social cybernetics:
In economics (contract theory):
Economic agency is an internal instrumentality through which external influences operate mechanistically on action. Internal agency events are a reflection of the impact of external environments from which causal attributes are ignored, and the self-system is simply a repository and conduit for environmental forces.
Emergent interactive agency defines Bandura's view of agencies, where human agency can be exercised through direct personal agency. Bandura formulates his view of agency as a socio-cognitive one, where people are self-organizing, proactive, self-regulating, and engage in self-reflection, and are not just reactive organisms shaped and shepherded by external events. People have the power to influence their own actions to produce certain results. The capacity to exercise control over one's thought processes, motivation, affect, and action operates through mechanisms of personal agency. Such agencies are emergent and interactive, apply perspectives of social cognition, and make causal contributions to its own motivations and actions using 'reciprocal causation'.
In social cybernetics:
Autonomous agency is able to embrace the concepts of both the economic agency and the emergent interactive agency. An autonomous system is self-directed, operating in, and being influenced by, interactive environments. It usually has its own immanent dynamics that impact on the way it interacts. It is also adaptable and (hence viable thus having a durable existence), proactive, self-organizing, self-regulating and so forth, participates in creating its own behaviour, and contributes to its life circumstances through cognitive and cultural functionality. Autonomous agency may also be concerned with the relationship between two or more agencies in a mutual relationship with each other and their environments, with imperatives for an agency's behaviour within an interactive context due to immanent emergent attributes.