Rudra is a mythological god of the Vedic Aryans. He finds mention in the Rigveda but there is a detailed section in praise of Rudra in the Yajurveda.
Rudra has two forms, ghora or terrible form and Shiva or auspicious form. Hence Rudra is one side and Shiva the other side of the same coin. They are not two, but one. The Brāhmaṇa text which interprets the Rudram states: “Rudra is verily this fire. He has two forms; one terrible, the other auspicious. If a man does sacrifice to Him by chanting the Shri Rudram, that man pacifies the terrible form”.