Who were the "giants" mentioned in the Bible?

I've come across several verses in the Bible that speak of "giants". For example: > **Gen 6:4** There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare [children] to them, the same [became] mighty men which [were] of old, men of renown. > > **Num 13:33** And there we saw the giants, the sons of Anak, [which come] of the giants: and we were in our own sight as grasshoppers, and so we were in their sight. > > **Deu 2:10** The Emims dwelt therein in times past, a people great, and many, and tall, as the Anakims; > **Deu 2:11** Which also were accounted giants, as the Anakims; but the Moabites call them Emims. However, I haven't been able to understand who exactly the giants were. Does this just refer to a race of men who were unusually large? Or were they not men but actually a separate species, and if so do we know anything of their origins?