From the KJV Bible,
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.
The sons of God are differentiated from sons of man because their height or stature was greatly different. They are the giants or Nephilim, as some versions (e.g. NIV) call it.
4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
I quote from Ellicott's commentary:
(2) The sons of God. . . . —The literal translation of this verse is, And the sons of the Elohim saw the daughters of the adam that they were good (beautiful); and they took to them wives whomsoever they chose. Of the sons of the Elohim there are three principal interpretations: the first, that of the Targums and the chief Jewish expositors, that they were the nobles, and men of high rank; the second, that they were angels. St. Jude, Jude 1:6, and St. Peter, 2 Ep., 2Peter 2:4, seem to favour this interpretation, possibly as being the translation of the LXX. according to several MSS. But even if this be their meaning, which is very uncertain, they use it only as an illustration; and a higher authority says that the angels neither marry nor are given in marriage. The third, and most generally accepted interpretation in modern times, is that the sons of the Elohim were the Sethites, and that when they married for mere lust of beauty, universal corruption soon ensued. But no modern commentator has shown how such marriages could produce “mighty men . . . men of renown;” or how strong warriors could be the result of the intermarriage of pious men with women of an inferior race, such as the Cainites are assumed to have been.
The Jewish interpreters, who well understood the uses of their own language, are right in the main point that the phrase “sons of the Elohim” conveys no idea of moral goodness or piety. Elohim constantly means mighty ones (Exodus 15:11, marg.). (Comp. Exodus 12:12, marg., Exodus 21:6; Exodus 22:8-9, where it is translated judges; Exodus 22:28, 1Samuel 2:25, where also it is translated judge.) In Job 1:6 the “sons of Elohim” are the nobles, the idea being that of a king who at his durbar gathers his princes round him; and, not unnecessarily to multiply examples, the “sons of the Elim,” the other form of the plural, is rightly translated mighty ones in Psalm 29:1.
When you read on, it says that the Sethites are among the descendants of Cain. They were boastful and advanced rapidly in the arts or army.
About the Giants, the commentary has this to say:
(4) Giants.—Heb., Nephilim, mentioned again in Numbers 13:33, and apparently a race of great physical strength and stature. Nothing is more probable than that, at a time when men lived for centuries, human vigour should also show itself in producing not merely individuals, but a race of more than ordinary height. They were apparently of the Cainite stock, and the text carefully distinguishes them from the offspring of the mixed marriages. The usual derivation of the name is from a root signifying to fall; but Lenormant (Origines de l’Histoire, p. 344) prefers pâlâ, which means “to be wonderful,” and compares the Assyrian naptû, “unique in size,” often found in the cuneiform inscriptions as the denomination of an ogre.
The same became mighty men.—Heb., They were the mighty men that were of old, men of name. “Gibborim,” mighty men (see Genesis 10:8), has nothing to do with stature, but means heroes, warriors. It is also generally used in a good sense. The children of these mixed marriages were a race of brave fighting men, who by their martial deeds won for themselves reputation.
So, the sons of God are from the descendants of Cain, who are great in stature (which can be known as Giants to inferior men), strength and they were noble/heroic. They are definitely not aliens or angels.