Genesis 4:1 to 2 - 1 Adam made love to his wife Eve, and she became pregnant and gave birth to Cain. She said, “With the help of the LORD I have brought forth a man.” 2 Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil. - NIV

Introduction
Genesis 4 leaves a lot to the imagination. Having a date for Adam that lines up with secular science however makes it easy to fill in the blanks. It helps answer questions such as: Who would kill Cain? Where did Cain find a wife? Who built Cain's cities? And so on. If Adam was not the first human, just the first human to know God (and the first human, with Eve, to disobey Him), then the supply of people to explain these events would be ample.
Cain and Abel are Born
1 Adam made love to his wife Eve,
and she became pregnant and gave birth to Cain.
She said, “With the help of the LORD I have brought forth a man.”
2 Later she gave birth to his brother Abel.
Now Abel kept flocks, and Cain worked the soil.
This brings us to our first blank: Was Cain Adam's and Eve's first born? Was Abel their second?
Verse 1 does not require Cain to be Eve’s first child; it does not require Abel to be his only brother. However, it does suggest that Cain was fully grown when Eve made her comment, and when Abel was born. Verses 1 and 2 do not exclude the existence of other siblings, or other people in general. In fact, this opening is rather silent on the matter.
What Did Eve Mean?
In verse 1b, Eve gives credit to the LORD, and refers to Cain as a man. Why not credit Adam? Why not call Cain her son or baby?
Amidst the explanations behind Eve's statement, there is a darkness there. Eve sinned in Genesis 3, and the extra pain of childbirth was a punishment of the LORD. Furthermore, we know Cain became a farmer of barren land, like his father. But, we also know he was jealous and had a temper, unlike his father. His father was weak; Cain was strong. Eve was resentful, and saw in Cain what was missing in Adam.
Determining a Time and Place for Cain and Abel
If Adam was born in 4336 BC and Seth in 4216 BC, this provides a bracket of time for Cain and Abel to be born and grow up. Scientifically, this period has been labeled as the Ubaid 3/4 period, which suggests that Cain took up dry farming east of the Tigris River, in the extreme North East of Iraq. West of the Tigris are the alluvial plains, which scientists believe were suitable for tending flocks...
"Adams suggests that conditions in the northern alluvial plans were drier [than in the south] and there much of the population tended flocks of sheep and goats and led a non- or semi-sedentary life."1
This suggests Abel's sheparding was between the Tigris and the Euphrates, though the distance from Cain is uncertain. Though drier than the south and hard on the body, there were benefits of Abel's lifestyle over Cain's: mobility, meat, a higher population of people with which to associate, and less need to contend with the cursed soil. Still, it was a hard lifestyle, and his ability to care for his flock amidst others would show in the fleece and the fat of his best.
Making Up
If Eve was indeed angry as Adam at the beginning of Genesis 4, there is some hope that having Cain kill her son Abel gave her some perspective... the perspective of losing a child...
25 Adam made love to his wife again, and she gave birth to a son and named him Seth, saying,
“God has granted me another child in place of Abel, since Cain killed him.”
26 Seth also had a son, and he named him Enosh. At that time people began to call on the name of the LORD.
Conclusion
Genesis 4:1-2 appear written to focus the reader (or listener) on the story that is about to follow... the story of Cain and Abel. By considering what we know of the period and Adam's and Eve's sin, it paints a picture of a weak husband raising up a strong son to take over the family craft (dry farming). This son would filling an emptiness in the mother's heart. Along comes a younger brother who went his own way -- raising livestock -- living out in the world, and succeeding. When God favored Abel's sacrifice over Cain's, his mother's resentment came out in his own rage, and Cain killed Abel.
In Cain, Eve sowed the seed of resentment against Adam and lost a son. More accurately, she lost two; Cain was not the man she thought, and now he had moved north. Scripture does not say whether she ever saw him again. So, when Adam made love to her and God granted her another son, we see a softer response. From "God has given me man," to "God has granted me a child."
You can tell Seth was raised up better than Cain, because when he himself had a son, no one was killed. People began calling on God's name.
1. "Ancient Mesoptomia: the eden that never was" - by Susan Pollack, 1999, p. 66
3/13/2012 - Added commentary on Genesis 4:25-26