Lord Shiva is one of the principal deities in Hinduism. He is a vital part of the trinity of Brahma, Vishnu, and Mahesh or Shiva. The primary role of Lord Shiva is said to be that of a destroyer, i.e., the one who destroys the whole creation (also called mahapralaya) at the end of a cosmic cycle or chaturyuga. However, this is a very surface definition of the role played by Lord Shiva in the creation. Actually, he is the destroyer of evil, negative energies, and vices. His role could be compared to that of a gardener who uproots the weeds from a garden, so that beautiful flowers can come out and blossom without any obstruction. Similarly, Lord Mahadeva eradicates the negative energies and evil tendencies from the mind of his devotees so that their lives blossom, and radiate with peace and bliss. One other aspect of his role as a destroyer is that he annihilates the virtually unending cycle of births and deaths, and grants Moksha to his devotees.
Lord Shiva is known by many different names; he has been referred with over one-thousand names in the puranas (the ancient Hindu scriptures). Some of the most popular names of Lord Shiva are Aashutosh, Bholenath, Gangadhara, Hara, Kailashadhipati, Lokapal, Mahadeva, Mahamrityunjaya, Nataraja, Omkara, Pashupati, Pushkara, Rudraksha, Sanatana, Shankara, Trilochana, Umapati, Vishveshwara, and Vishwanath, to name a few.
Lord Shiva is called Mahadeva, or the greatest god, primarily due to his highly benevolent and merciful nature. He does not look at the background and attributes of his devotee; whoever calls him with an earnest heart, be it a king or beggar, sinner or saint, human or devil, Lord Siva easily gets pleased with him and grants a varam or boon, hence the name Aashutosh.
The mythological story of samudramanthan is known to many people. When the devas and asuras churned the mighty ocean, certain objects appear from the ocean such as Kamadhenu, Airavata, Ratnas, Apsaras, Parijat, and several others. All the objects were distributed among the gods and devils as per mutual consent, though there was a great struggle happened to obtain the amrita or the nectar of immortality, which was later resolved with the aid of Lord Vishnu. However, when the halahala or the lethal poison emerged from the ocean, everybody started to run hither and thither; nobody wanted to share it or find a solution to it, whether they were devas or the asuras. In such a situation, when that life-threatening poison started to spread, everybody came to Lord Shiva and prayed to him to save them and the entire creation. At that juncture, the gracious Lord Shiva accepted their prayers and drank that poison as if it was nectar. He was known by the name of neelkantha thereafter, owing to his bluish colored throat that happened due to the effect of the poison (Siva stored the poison in his throat). That is to say, Lord Shiva can go to any extent to save his devotees.
The night of the worship of Lord Mahadeva is celebrated as Mahashivratri festival by Shiva devotees around the world. It is celebrated on the Krishna paksha Chaturdashi tithi of the Phalguna month; usually in the month of February. According to the Hindu mythology, the marriage of Shiva and his divine consort, Parvati or Uma had happened on the day on which Mahashivaratri is celebrated. It is said that the devotee who prays to Lord Mahadeva with a pure heart and performs abhisheka on Mahashivratri is granted the greatest of wishes by the almighty Lord Shiva.
Though there are many hymns, prayers, and mantras, recited to perform Shiva pujan, one mantra, called the Mahamrityunjaya, gains precedence over all others. The same is given with its meaning below:
Mantra
Om Tryambakam Yajaamahe
Sugandhim Pushti-Vardhanam
Urvaarukam-Iva Bandhanaan
Mrtyor-Mukshiiya Maa-Amrtaat
Meaning
Om, We Worship the Three-Eyed One (Lord Shiva),
Who is Fragrant (Spiritual Essence) and Who Nourishes all beings.
May He severe our Bondage of Samsara (Worldly Life), like a Cucumber (severed from the bondage of its Creeper),... and thus Liberate us from the Fear of Death, by making us realize that we are never separated from our Immortal Nature.
Really, the mercy and the glory of Lord Shiva are boundless like him! Glory to the god of gods! Glory to the great Lord Mahadeva again and again!