In 'Sonnet 18' by Shakespeare, who is the narrator describing?

I have read on some websites that the sonnet is describing a woman, on others it says that it is describing a man. In some it calls the person 'the youth' and in some it calls the person 'the fair lord', on one website it even says that it is the actual poem he is describing, not a person. I am writing an essay about this sonnet, and I don't know how to refer to the person being described. I have tried just calling them a person, for example "this suggests that the person described is beautiful", but it doesn't really work the whole way through the essay!! What should I refer to them as?