The literal meaning of the word "Maya" is illusion. Any thing which is added to the reality is illusion or Maya, and this veiling power of illusion is able to create the difference between 'me' and 'mine', or 'thee' and 'thine'. Maya creates the ignorance of the individual self. More on the concept of Maya.
So Maya Shakti is the divine power which brings out the evolution of this present world. It makes this possible by an interplay of the three gunas : sattva, rajas and tamas. Cosmic Consciousness becomes individual consciousness by its own maya.
In Hindu mythological literature, Maya has been described in many contexts. Apart from Maya Shakti, Maya also stands for a demon that was the architect of the demons and the creator of the magical arts.