Mainform 2 :

Siva in this form is known as Nataraja, the cosmic dancer. He dances in the centre of the universe. In this form Lord Siva is depicted in his famous Ananda Tandava dancing pose, with one leg in the air and he has four arms. Nata means dance and raja means king. Thus he is known as king of the dance.
The movements are very elegant and smooth but it can also be very violent. The steps will the soften the blow of the devotees. The movements can also be the Tandava dance. This is the dance of destruction and the dance on places where people are cremated.

At the moment that Shiva stops playing the drum to find a better rhythm the universe will be destroyed. The moment when he starts dancing again the new universe will be created.

He is encircled by a ring of flames and is holding a drum. One of his hands is in the Abhaya Mudra posture (“Fear not, I will protect you”) the ring of flames are the purest form of energy, but it is also the cremationfire. This fire/energy symbolizes the holy mantra AUM, the Base Sound for Creation.
The Sound of the drum symbolizes sound as source of Creation and god as source of sound. (Nada-Brahman)

Other forms of dancing Shiva are:

Lalatatilakam:
Shiva has in this form 8 up to 16 arms, with one leg on a dwarf named Apasmara, who is said to represent ignorance or false ego, which makes souls forget their true identity. Nataraja destroys false ego.

Tandava:
Shiva in this form has 10 arms, he dances in ecstasy on cremationplaces.
He is often accompanied with Devi and ghosts.

Gangadhara- Murti:

In this form Shiva normally stands and has 4 arms.
Gangadhara, means one who bears weight of the Ganges River as it falls to earth from the heavens. When the Ganges descended to earth, it fell on his head and through his jata (matted hair), forming many small streams. Eventually these streams combine and form the Ganges at Haridwar.
Parvati often stands next to him looking to Ganga and is jealous.
She has probably all reason to be jealous because Shiva is often depicted with a phallus in erection.