What To Do For The Discoloration In The Skin?

HI Doctor, my age is 25, height 5'7" & weight is 56.
I have a Skin related problem that I have been suffering this for 10 to 12 years. My hand chest & back became black, I do not know what I call this. Is this a kind of disease or disorder.
Please Kindly help me out.....
Awaiting for your reply.