The greatness of Jyotirlingas and their Upalingas - Shiva Purana

In Santana Dharma, Shivalinga is supreme, formless and cosmic cause of this universe. Shiva is assumed form of Shivalinga.   A Jyotirlinga or Jyotirling or Jyotirlingam i.e., lingam of light, is a shrine where Lord Shiva appeared. Such Jyotirlinga are twelve. These Jyotirlinga of Lord Shiva are the holiest of holy shrines of Lord Shiva. Jyotirlinga are the Twelve special shrines dedicated to Lord Shiva since ancient times.
There are many names of Shiva in which 108 names are found in Puranas, but know the names here- Mahesh, Neelkanth, Mahadev, Mahakal, Shankar, Pashupatinath, Gangadhar, Natraj, Trinetra, Bholenath, Adidev, Adinath , Trimbak, Trilokesh, Jatashankar, Jagdish, Pralayankar, Vishwanath, Vishveshwara, Hara, Shivshambhu, Bhootnath and Rudra.
According to Shiva purana, Brahma, Maya, Jiva, Mind, intellect, mind, ego, sky, air, fire, water and earth are called Jyotirlinga or Jyoti Pind.
Spiritual Significance:
Those who see Shiva's life and philosophy with real vision are Shiva devotees with the right intelligence and catching the reality, because Shiva's philosophy says live in reality, live in the present, do not fight with your mind, look at them as strangers and also use imagination for reality. Before Einstein, Shiva had said that imagination is more important than knowledge.
Shiva has given darshan to people in every era. Shiva was also there during the time of Rama. There was Shiva even in the Mahabharata period and there is a mention of Shiva's darshan in the time of Vikramaditya. According to Bhavishya Purana, Lord Shiva also appeared to King Harshavardhana.
What is Jyotirlingas:
The Jyotirlinga is an aspect of the Supreme Shiva, in his formless form. He is the Lingodbhava, the one who arises from the Linga and appeared as a fiery column of light. The Jyotirlinga temples are the places where Shiva took a form as that radiant column of light. The Jyotirlingas are considered to be the most sacred among the Shiva shrines. The Jyotirlingas' are filled with a divine light, radiance or Jyoti. The term "Jyotirlinga" thus means the radiance of the Almighty God, Shiva. It is believed that the divine effulgence emanated by these lingams can be seen only by those who have attained a high state of spiritual enlightenment.
According to the Shiva Mahapurana, Brahma and Vishnu once had an argument over who among them was Supreme. In order to end their debate, Shiva pierced the three worlds, taking the form of a massive Infinite Pillar of Light (Jyotirlinga).
According to Vayu Purana, in the Holocaust, all creation which gets absorbed and which appears in the reincarnation period is called Linga. In this way, the entire energy of the world is the symbol of gender. This whole universe is a point-sound form. The point is Shakti and Nad Shiva. Point means energy and sound means sound. This is the basis of two whole universe. For this reason, worship of Shivling is a symbol.
Where located:
The twelve Jyotirlingas are:
Someshwara in Saurashtra (Near Veraval, Gujarat), Mallikarjuna in Srisailam (Andhra Pradesh), Mahakaleshwara in Ujjain (MP), Parmeshvara in Omkara (Khandwa, near Indore, MP), Kedarashwara on the Himavat (Garhwal Himalayan range, Uttrakhand), Bhima Shankara in Dakini (In Sahyadri hills near Pune, Maharashrtra), Vishwashwara in Varanasi (UP), Tryambakeshvara on the bank of Gautami (Near Nashik, Maharashrtra), Vaidyanatha in the cremation ground (Deodhar, Jharkhand), Nagashwara in the Daruka Forest (Near Dwarka, Gujarat), Rameshwara at Setubandha (Tamil Nadu) and Ghushmeshwara at Shivalaya (Near Ellora, Maharashrtra). The Upalingas of Jyotirlingas are:
The Upalinga of Someshwara is called Antakesha and it is installed at the confluence of the river Mahi and the ocean. The Upalinga of Mallikarjuna is glorified as Rudreshvara. It is pleasing and is stationed in Bhṛgukakṣa. The Upalinga of Mahakala is well known as Dugdhesha. It is famous in the region of Narmada and is remembered as destructive of all sins. The Upalinga of Oṃkara is famous as Kardamesha in the Bindu lake and yields the fruits of all desires. The Upalinga of Kedareshvara is Bhutesha on the banks of the Yamuna. It is destructive of sins of the visiting devotees. The Upalinga of Bhimashankara is Bhimeshvara. It is famous in the Sahya Mountain and it causes the increase of great strength. The Upalinga of Nageshwara is Bhuteshvara. Installed on the banks of Mallikasarasvati, it quells sins by the very sight. The Upalinga attached to Rameshwara is Gupteshvara. The Upalinga attached to Ghushmesha is Vyaghreshvara.  
Why Worshipped:
He who reads or recites the twelve names early in the morning is freed from all sins and attains the fruits of all Siddhis. Those who recite them without any desire but with purity of mind will never have rebirth involving residence in the mother’s womb. By worshipping these, people of all castes get their miseries destroyed in this world and become liberated hereafter, to be sure. The food offerings of these Jyotirlingas shall be taken and others shall be fed with the same. Such acts reduce the sins to ashes instantaneously. Even if a person of base birth happens to see Jyotirlinga, he will be born in a pure and good family.
How Originated:
Kotirudra Saṃhita of Shiva Purana deliberates on the greatness and origins of Jyotirlingas and their Uplingas. The entire universe is in the form of a phallus (Linga). In Sanskrit, Linga means a "mark" or a symbol, which points to an inference. Formless Shiva is prayed as Lingam (Nirankar). Thus, the Shiva Linga is a symbol of Lord Shiva.
Nothing other than Shivalinga (Phallic image) has real existence. There is no limit to the number of phallic images. The holy centres are equipped with phallic images. The three worlds consisting of the gods, Asuras and human beings are pervaded by Shiva in the form of phallic image for the welfare of the worlds. Shiva is worshipped by the gods, Asuras and human beings.
In order to bless the worlds, Shiva assumes different phallic forms here in the holy centres as well as in other places. Shiva incarnates in those places wherever he is devoutly remembered by the devotees. After carrying out their task he stations himself there. He has assigned his phallic form for the benefit of the worlds. The devotee attains perfection by worshipping that form.
 The legends of various Jyotirlingas are mentioned in ensuing articles to be published as per Shiva Purana.
 
Reference: Shiva Purana, Koṭirudra Saṃhita, Chapters – 1.