In Genesis chapter 6 and Numbers chapter 13, you read about the "sons of God", "giants" and the "Nephilim" and you may wonder if these terms refer to human or supernatural beings.
"And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. 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." (Genesis 6:1-4)
“We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them” (Numbers 13:33). This scripture makes me wonder if Goliath was one of the Nephilim.
These three terms describe the same entities, ie: giants. Some people believe that these giants refer to the descendants of Adam's sons, Seth and Cain.  Other people believe the Nephilim (giants) were the offspring of human women and fallen angels, because the term "sons of God" used in the Book of Job and the reference to fallen angels in the Book of Jude, suggest these children were fathered by supernatural beings. 
- "Now there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them." (Job 1:6)
- "Again there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them to present himself before the Lord." (Job 2:1)
- "And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day." (Jude verse 6)
Another view regarding who the "sons of God" were is that they were human men, possessed by demons, that passed on their demonic DNA into the human race. If true, it may be one of the reasons why God had to destroy everyone on Earth except Noah and his family in order to preserve the bloodline of Jesus. “These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.” (Genesis 6:9).
Another curious phrase from Genesis 6:4 is: "There were giants in the earth in those days; and also after that,..." If every living thing except Noah and his family were wiped out in the flood, where did the next generation of giants (Nephilim) come from? Hugh Ross (astrophysicist) explains how this might have happened in his excellent article: "How Did the Nephilim Reappear after the Flood?"  In this article he explains that other fallen angels separate from those who were sent to the bottomless pit by God mated with human women after the flood.
Regardless of what you believe about this subject, it makes for interesting reading and I'm looking forward to reading new material about it. It's unsettling to think that the Nephilim could be in our world today doing the same thing.