What exactly is the Susano'o technique? The Uchiha and any Sharingan user who awakens the double Mangekyou Sharingan have the ability to use the Susano'o technique, and each Sharingan user seems to have a unique Susano'o form.
But what exactly is the Susano'o? Is it a deity, a special form of invocation or just a materialized form of chakra? Has this been explained in the manga so far?
Susanoo is an ability granted to those who awaken the powers of both their Mangekyō Sharingan. It creates a gigantic, humanoid being that surrounds the user and fights on their behalf. As one of the strongest techniques granted to those that have acquired the Mangekyō Sharingan, it is the user's guardian deity, but at the same time, it consumes the user's life force and damages their body over time

The answer to your question is : Susanoo is a materialized form of user's chakra.


  According to Sasuke Uchiha, if a Sharingan user unlocks two Mangekyō
  Sharingan powers, one in each eye, known as the "Double Mangekyō"
  (ダブル万華鏡, Daburu Mangekyō), it gives access the third power, Susanoo.
  Nevertheless, for a Sharingan user to activate Susanoo, according to
  Obito Uchiha, is a rarity. Since it is formed through the
  materialisation of the user's chakra, its abilities, appearance and
  colour vary among individuals. While the user usually remains
  grounded within Susanoo, it is possible for them to freely move around
  within its body, as well as levitate inside of it. The user can allow
  others to enter Susanoo, or leave its body entirely. While Susanoo
  acts as a defence against external attacks, anything coming out of
  Susanoo will simply phase through it.


Source : Naruto wikia : Susanoo