Overview of Political Process

"... why am I so interested in politics? If I were to answer you very simply I would say this: why shouldn't I be interested? That is to say, what blindness, what deafness, what density of ideology would have to weigh me down to prevent me from being interested in what is probably the most crucial subject to our existence, that is to say the society in which we live, the economic relations in which it functions, and the system of power which defines the regular forms and regular permissions and prohibitions of our conduct? The essence of our life consists, after all of the political functioning of the society in which we find ourselves." Michel Foucault 
From Wiki -

Since the 1790s, the country has been run by two major parties. The United States does not have a parliamentary system, in which governing coalitions are formed after elections, so coalitions are formed before elections under the umbrella of the party organizations. In the absence of a parliamentary system, third parties cannot thrive. Since the Civil War, the two major parties have been called the Republican and Democratic parties. Many minor or third political parties appear from time to time. They tend to serve a means to advocate policies that eventually are adopted by the two major political parties. At various times the Socialist Party, the Farmer-Labor Party and the Populist Party for a few years had considerable local strength, and then faded away. At present, the Libertarian Party is the most successful third party.
Most officials in America are elected from single-member districts and win office by beating out their opponents in a system for determining winners called first-past-the-post—the one who gets the plurality wins, (which is not the same thing as actually getting a majority of votes). This encourages the two-party system; see Duverger's law.

Another critical factor has been ballot access law. Originally, voters went to the polls and publicly stated which candidate they supported. Later on, this developed into a process whereby each political party would create its own ballot and thus the voter would put the party's ballot into the voting box. In the late nineteenth century, states began to adopt the Australian Secret Ballot Method, and it eventually became the national standard. The secret ballot method ensured that the privacy of voters would be protected (hence government jobs could no longer be awarded to loyal voters) and each state would be responsible for creating one official ballot. The fact that state legislatures were dominated by Democrats and Republicans provided these parties an opportunity to pass discriminatory laws against minor political parties, yet such laws did not start to arise until the first Red Scare that hit America after World War I. State legislatures began to enact tough laws that made it harder for minor political parties to run candidates for office by requiring a high number of petition signatures from citizens and decreasing the length of time that such a petition could legally be circulated.
It should also be noted that while the overwhelming majority of elected officials do identify with a political party, the political parties of the United States are much more individualistic than in other political systems (i.e. in a parliamentary system). More often than not, party members will "toe the line" and support their party's policies, but it is important to note that they are free to vote against their own party and vote with the opposition ("cross the aisle") whenever they please.

"In America the same political labels—Democratic and Republican—cover virtually all public officeholders, and therefore most voters are everywhere mobilized in the name of these two parties," says Nelson W. Polsby, professor of political science, in the book New Federalist Papers: Essays in Defense of the Constitution. "Yet Democrats and Republicans are not everywhere the same. Variations—sometimes subtle, sometimes blatant—in the 50 political cultures of the states yield considerable differences overall in what it means to be, or to vote, Democratic or Republican. These differences suggest that one may be justified in referring to the American two-party system as masking something more like a hundred-party system."

Youth Guide to Politics, Part One: What Are Political Action Committees? 

http://www.wiretapmag.org/rights/43192/ 

About PACs - Political Action Committees

http://usgovinfo.about.com/od/thepoliticalsystem/a/aboutpacs.htm

List of Political Action Committees via Wiki
http://en.wikipedia.org/wiki/List_of_political_action_committees

In the 2008 elections, the top 9 PACs by money spent by themselves, their affiliates and subsidiaries were as follows:

1. IBEW PAC $3,344,650
2. AT&T Federal PAC $3,108,200
3. American Bankers Association (BANK PAC) $2,918,14
4. National Beer Wholesalers Association PAC $2,869,000
5. Dealers Election Action Committee of the National Automobile Dealers Association $2,860,000
6. International Association of Fire Fighters $2,734,900
7. International Union of Operating Engineers (IUOE) Political Education Committee $2,704,067
8. American Association for Justice PAC $2,700,500
9. Laborers International Union of North America (LIUNA) PAC $2,555,350

http://en.wikipedia.org/wiki/Political_action_committee