Nandi is the white bull which Shiva rides, and the leader of the Ganas. The white color of the bull symbolizes purity and justice.
Nandi is the name for the bull which serves as the mount (Sanskrit: Vahana) of the god Shiva and as the gatekeeper of Shiva and Parvati. In Hindu Religion, he is the chief guru of eighteen masters (18 Siddhar) including Patanjali and Thirumular. Temples venerating Shiva display stone images of a seated Nandi, generally facing the main shrine. There are also a number of temples dedicated solely to Nandi.
The application of the name Nandi to the bull (Sanskrit: vṛṣabha) is in fact a development of recent centuries, as Gouriswar Bhattacharya has documented in an illustrated article entitled "Nandin and Vṛṣabha". The name Nandi was earlier widely used instead for an anthropomorphic deity who was one of Shiva’s two door-keepers, the other being Mahākāla. The doorways of pre-tenth-century North Indian temples are frequently flanked by images of Mahākāla and Nandi, and it is in this role of Shiva’s watchman that Nandi figures in Kālidāsa’s poem the Kumārasambhava.