Q:

What is the significance of jyotirlinga?

What is the significance of Jyotirlinga?
Are these temples or just ling placed on ground?
Also why there are only 12 Jyotrilinga? Also at specific places only?

A:

The significance is that all the 12 Jyotirlingas are Swayambhu or self- manifested or self- existent. They are not man made or krittima. So, they represent Lord Shiva fully. And their greatness is unmatched.
In MatrikA Bheda Tantram, Lord Shiva says that in Kailasa there exists a Jyotirmaya lingam (or Jyotirlingam ) ( which is of course inaccessible to human beings ) , similarly on earth  a corresponding lingam exists in Visweswara KAsi. And, similarly there 11 others at different places of India.
We can't see the most divine lingam that is there on the Kailasa mountain but we can definitely see ( & pray to ) the lingam ( that is in KAsi ) which Lord Shiva says as being representing the one-sixteenth potency of the Kailasa lingam. 
So, this gives us a rough idea of the divinity and the unmatched greatness of the Jyotirlingams that are found earth and in particular that of the KAsi lingam.

Sri Shiva Uvacha:
YathA Jyotirmayam Lingam KailAsha Shikhare Mama | Tasyaiva
  ShodashAnsaikah KAshyAm Visweswarah Stithaha ||
Lord Shiva said:
( Hey Parameswari ), Just like there is a Jyotirmaya Lingam on top of
  Kailasa there is a Lingam at Visweswara Kasi , which is of one
  sixteenth potency of the lingam at Kailasa.
MAtrikA Bheda Tantram, Chapter 8, Verse 4.

Are these temples or just ling placed on ground?

These lingas came first, then came the temples and holy kshetras associated with them. So, the temples were built later.
The stories associated with these Jyotirlingams are found in several Puranas and i have seen questions and answers on quite a few of them on this site. So you can do a search.
Now, as to why there are exactly 12 such Jyotirlingams on earth  and not one less or one more , i can't answer. But even i want to know the reason.

A:

The significance of the Jyotirlingas is described in Chapter 1 of the Koti-Rudra Samhita of the Shiva Purana. 
The sages ask Suta to describe the glory of the Shivalingas. Suta replies that there are Shivalingas all over the three worlds as Shiva is worshiped by everyone be it gods, demons or humans. 
The Shivalingas are consecrated to shower grace upon the devotees. Suta goes on to say that whenever the devotees remembered Lord Shiva he appeared there and established himself in the form of a linga. 

सर्वेषां शिबलिङ्गंन्धां’मुने! सङ्खद्या न विद्यतॆ। सर्वालिङ्गययीभूमि:
  सर्वंलिङ्गग्रयंजगतू॥९॥
O Sage, there are innumerable Sivalingas in the world. The entire earth
  is full of lingas, besides the entire earth has the form of linga.
यत्र यत्र यदा शम्भुंर्भक्त्यां भक्तेश्च संस्मृतः। तत्र तन्रावतीर्याथ
  कार्यं वृभ्त्वम्या स्थितस्तदा॥१६॥
Wherever Siva was remembered with
  devotion by the devotees, he appeared at such places and after
  performing the task of his devotees, he established himself there.
लोकप्ताप्रुक्कारार्घं स्वलिङ्गं चाप्यवल्मयत्। तल्लिङ्गं पूजयित्वा तु
  सिद्धि समधिगच्छति॥१७॥
In order to do good to the people, he created
  the linga and by adoring the lingas, the devotees meet with success.

Since there are innumerable lingas all over the world Suta describes the most prominent ones to the sages which constitute the 12 Jyotirlingas.
Out of all the lingas in the world these 12 are the most auspicious and bestow the highest merits. The merit that these Shivalinga's bestow cannot be described by Brahma himself. 

पृथिव्यां यानि लिङ्गानि तेषां सङ्खद्या न विद्यते।
  तथापिचप्नधानानिकय्यन्तेचमयाद्विजा:।।१८॥
O Brahmanas, all the lingas of Siva found on earth, are beyond
  counting, still I would like to describe the prominent lingas.
ज्योतिर्लिङ्गानि यानीह मुख्यमुख्यंम्यानि सत्तम!। तान्यहं कथयाम्यद्य
  श्रुत्वा पापं व्यपोहति॥२०॥
O Excellent one, all the prominent Jyotirlinga are being described by
  me. By listening to their names all the sins are washed out in a
  moment.
सौराष्टे सोमनाथंच श्रीज्ञैले मल्लिकार्जुनम्। उज्जयिन्यां
  महक्कालमोड्क़रे परपेश्वरम्॥२१॥
केदारं हिमवल्पृष्ठे ङाकिन्यां भीमशङ्करम्। वाराण्स्यां च विश्वैशं
  त्र्यम्बकं गौतमीतटे॥२२॥
वैद्यनाथं चिताभूमी नागेशं दारुकावने। सेतुबबन्ध च रामेशं घुश्मेशं च
  शिवालये॥२३॥
There is Somanatha in Saurastra, Mallikarjuna at Srisaila, Mahakala in
  Ujjaini, Paramesvara or Amalesvara at Omkara.
Kedarasvara is in Himalaya, Bhimasankara in Dakini, Visvanatha at
  Varanasi, Trayambaka at the banks of the Gautami 
Baidyanatha at the cremation grounds, Nagesvara at Darukavana,
  Rameshvara at Setubandha, while Ghusmesvara or Ghusnesvara at Sivalaya
द्वादशैतानि नामानि प्रातरुत्थाय य: पठेत्। सर्वपापविनिर्मुक्ता
  सर्वसिद्धिफलं लभेत्।। २४ 
A person who recites the name of these twelve
  Jyotirlinga daily in the early morning, he is relieved of all the
  sins, and he meets with all the success.
यं यं कापग्मपेक्ष्यैव पठिष्यन्ति नरोत्तमा:। प्राएयन्ति कामं तं तं हि
  परत्रेह मुनीश्वरा॥२६॥ 
O Excellent sage, whosoever will recite the names of these Jyotirlinga with 
  any particular desire in his mind, he will achieve success in that 
  particular field in the present as well as in  the future world.
ज्योतिषां चैव लिङ्गानां व्रह्मादिभिरलं द्विजा:। विशेषत:फलंबक्तुंशक्यतेनपरैस्तथा॥२९॥ 
O Brahmanas, the result of the worship of
  the Jyotirlinga cannot be described by Brahma and others.

