Reason.com has a really good image for detailing the immigration/naturalization process. This image was created in 2008, there have been no significant changes to immigration/naturalization law since then: http://reason.com/assets/db/immigration-flow-chart.jpg

One salient nugget: “Sorry! There is virtually no process for unskilled immigrants without relations in the US to apply for permanent legal residence. Only 10,000* green cards are allotted every year, and the wait time approaches infinity. (Those who receive H-2A or H-2B temporary visas for seasonal work cannot transition to a green card.)”

* This number has since been increased to 50,000, which is determined by lottery, but still represents only 5% of the total number of newly minted legal permanent residents per year.