Your question is paradoxical. You say you think you have Dissociative Identity Disorder (DID) and yet you ask what is it.
Let me first inform you that what is commonly perceived as DID, is actually Schizophrenia in most cases. Secondly, DID is almost non-existent in India. There have been anecdotal reports of the disorder in the western world, mostly the US. But since the features of the disorder have been something exciting and curious for the media and film makers across the globe, it has caught people's imagination.
In the west too, modern research tells that, many cases first diagnosed as DID, are not really that. However there are always rare instances and the basic feature of the disorder is that the afflicted individual takes multiple identities without his/her conscious awareness of the same and exhinits personality traits of the person he/she has taken the identity of, in a particular episode, without the memory of such episodes. The defense mechanism called 'Dissociation' is at use during this episode, where the person's psychic mechanism, to defend the ego adopts this alter ego.
I'm sure this is beyond the purview of your understanding and it would be foolhardy to try and understand this through this medium.
I, therefore recommend you to see a Psychitric professional, in person, who can evaluate in detail, whatever it is that you are going through.