Power is almost always reciprocal, but usually not equally reciprocal. To control others, one must have control over things that they desire or need, but it is rare to hold that control without there being a measure of reverse control also, which may be larger, smaller, or equal. For example, an employer usually holds considerable power over his workers because he holds control over wages, working conditions, and who is permitted to remain employed. His workers, however, hold some reciprocal power: they may leave, work more or less diligently, group together to form a union, and so on.
Because power is both relational and reciprocal, sociologists speak in terms of the balance of power between parties to a relationship: all parties to all relationships have some power: the sociological examamination of power is concerned with discovering and describing the relative strengths, which may be equal or unequal, stable or subject to periodic change. Relationships in which the parties are relatively equal or nearly equal are more usually analysed in terms of constraint rather than power.
Even in structuralist social theory, power is regarded as a process, an aspect to an ongoing social relationship, not a fixed part of social structure.
Violence or other kinds of force can be used to exercise power (this is called coercion). In general, however, power is exercised more peacefully.
The thought of Friedrich Nietzsche underlies much 20th century analysis of power. Nietzsche disseminated ideas on the "will to power", which he saw as the domination of other humans as much as the exercise of control over one's environment.
One of the broader modern views of the importance of power in human activity comes from the work of Michel Foucault. Feminist analysis of the patriarchy often concentrates on issues of power: note the "Rape Mantra[?]": Rape is about power, not sex.