Vishnu is the part of the Hindu Triumvirate or Trimurti. Like other two Lords - Shiva and Brahma, Lord Vishnu is immortal. The part of the immortality can also be attributed to the fact that all the Devtas partook the Amrut or the holy nectar that granted them immortality.
Lord Vishnu plays an important role as the protector of the universe and often returns to earth to protect against troubles and problems. He helps to reduce the trouble on the earth and restore the balance between good and bad. His incarnations often have served their purpose on the earth and it is believed that He will again reincarnate on the earth to save it in 2025 CE.