Yoni, the creative power, is a part of the shiva linga, which represents Shiva and Shakti. The base of the linga is the yoni, representing the female sex organ. Shiva linga symbolizes the union of creative energies of Shiva and Shakti, or male and female.
Tantric teachings do not consider yoni just as a female sex organ, but associate it with the Divine power. Shiva, the god of destruction, is all-powerful only when he is with Shakti, the source of life or creation.