Mainform 5 :

In this form he is the destroyer and he is the power who preserve the cycle of destruction and creation. He destroys all shapes and by destroying he releases the energy which is used to create again.

As destroyer Shiva has many forms:

Gajasura-murti:
In this form Shiva dances on the head of an elephant-demon.
This demon disturbed a ritual of the Brahmin priests dedicated to Shiva.
Shiva stepped out of the lingam, chopped off the head of the demon and worn the skin of the demon as a cloth.

Tripurantaka-murti:
In this form he carries a bow and arrow and beats the 3 cities of Asura.

Kalari-Murti:
In this form he beats Yama/ Kala the god of the underworld.
Shiva stepped out of the lingam to beat Yama. Yama came to get a person who was in deep meditation in front of the lingam of Shiva.

This form is depicted as Shiva stepping out of the lingam with one foot on the chest of Yama.

Next to the 5 Main forms there are other forms of Shiva:

Ardhanarishvara:
This is the androgyny form of Shiva, half man and half woman.

Mahadeva:
The Supreme God.
The three gods of the Trimurti are seen as forms of Shiva.

Sadashiva:
This is the eternal Shiva or Panchanana (with the 5 faces).
Shiva in this form has 5 faces and 10 arms.
He symbolizes Creation, Preserver, destruction and Salvation.
The 5 faces symbolize the 4 winds and the heaven or the 5 senses.

Mahakala (the great time):
Shiva in this form stands on a lion.