Shakti is a Hindu religious concept that means feminine power. Although it is considered feminine in nature, it is usually reserved as a term used to discuss the power of Hindu goddesses, rather than human females. Shakti is divine, or sacred feminine power. The sect of Hinduism that devotes to the goddess is known as Shaktism, and a member of this sect is called a shakta.
Shaktism is not the only sect within Hinduism that acknowledges the power and importance of the goddess. Other sects such as Vaishnavism and Shaivism also have a place for shakti. However, in these other sects, it is thought of as a complimentary power to that of the corresponding god. In other words, goddesses are the consorts of gods, and thus their power is important as a compliment to the male power, virya.
To someone who devotes to Vishnu, or Shiva, shakti is ability, life force, creative, and procreative power. Virya, the power of the male god, is authority, energy that tames, directs, constructs, and gives purpose to the shakti. However, for a shakta, the shakti of the goddess may take a more important role outside of its association with the male force. Within Shaktism, it is the true power behind the male god, an abstract and all-pervasive power.
The semantic difference here is subtle. Perhaps the easiest way to distinguish Shaktism is to say that it devotes to the goddess and her shakti above all else. This concept is more appropriately called Adi Shakti, or the ultimate shakti. It is not just feminine divine power, but the ultimate feminine power behind all creation.
When we talk about the goddess and her shakti, there are a few things which require elaboration. In the first place, this goddess can generally be considered Maha Devi, or the great goddess. Shakti is, of course, also a power held by and associated with the many manifestations of Maha Devi, such as Durga, Kali, Parvati, Saraswati, Lakshmi, and so on.
The concept should be understood as part of a three-fold power, the three-fold nature of the goddess. Shakti, again, is divine power and creative energy. It works in tandem with maya and prakrti. Maya is illusion, the illusory power of the goddess. Shakta theology sees this as a positive, creatively potent illusory power, rather than a crafty, deceptive power. Prakrti is nature, or the material manifestation of maya and shakti. Prakrti is both the body of the goddess, and all other material forms, as these are part of her body.
In addition to the concepts of shakti, maya, and prakrti, a fourth concept comes into play in terms of the nature and function of a goddess. This fourth concept is líla, or divine play. The purpose of líla is to bring pleasure through play. The four aspects of the goddess work together in the following way: The goddess creates (shakti) all the material forms (prakrti as the goddess herself) with the illusion that these forms have an existence apart from her (maya) for the purpose of her own divine pleasure (líla).