What Causes Blotchy, Purple Coloured Skin On Hand?

I'm 15 and female. On the back side of my left hand there are patches of blotchy, purple coloured skin which I have had for atleast a year now. I'm not sure what it is, however I have noticed some elderly people have something maybe not the same but similar on their arms/hands or legs.>What might this discolouration be, and how can I get rid of it?