Who is Shiva?
According to scriptures, Lord Shiva is god who was residing in Himalayan Mountains. Parvati was his wife and Ganesh and Kartik were his sons. These are physical attributes but in true sense any human being who attains 'self realization' achieves the internal state of 'Shiva'.
Why is he also known as Neelkanth?
The essence behind the word "Neelkanth" is that one who has swallowed all the poisons (abuses and insults) given to him in this world with equanimity and yet offered blessings to others, hence becoming Shiva.
The ultimate transition from Om Namah Shivay to Shivoham Shivoham...
In the ignorant state (Jiva Dasha) one considers himself separate from Shiva and says 'Om Namah Shivay' (I bow to Shiva), but after attaining self realization one gets into constant awareness of soul which is separate from the body and then he experiences Shivoham, Shivoham...(I am Shiva, I am Shiva). I am a soul and not the body. Thereafter, there remains no distinction between Shiva and my true self.
Param Pujya Dadashri achieved self realization (experience of the soul) in June 1958. He used to impart the same 'self realization' to others in just 2 hours. Pujya Dr. Niruma attained self realization from Pujya Dadashri in 1968.
You too can experience self realization by attending 'Gnanvidhi' from Pujya Deepakbhai. Thereafter by observing five Agnas (fundamental principles) would lead you to ultimate salvation.