Believe it or not, political parties do serve a purpose besides triggering 95% of all arguments ever started since time began. They help define potential voters' beliefs and positions on important issues and use that strength in numbers to get candidates who share those views elected into office. Currently, the United States has two major political parties, the Democratic party and the Republican party. There are also various third parties, but their political clout is generally considered marginal at best.
Roughly speaking, political parties serve the same purpose in the political world as denominations or sects serve in the religious world. If an individual were the only person alive on an island, his or her word would be considered law. Once a second person arrives, however, differences of opinion would almost inevitably arise. As more and more people arrive, these differences could divide inhabitants into different camps of thought. Living together on the island would still be a priority for all, but differences of opinion on how to accomplish that goal would always exist.
On a larger scale, political parties would represent all of those different camps of thought. Some parties are composed of very liberal members who believe a larger national government has an obligation to provide essential services for all citizens. Others are composed of very conservative members who believe a smaller national government should allow the private sector to provide such assistance. It is political differences such as this which create the need for at least two political parties, one more progressive and the other more conservative in nature.
Political parties also help to define specific issues and positions which go along with their broad stroke conservative or liberal leanings. The Democratic party, for example, can specifically take an official stance on an controversial issue, such as abortion rights. The Republican party's stance may be pro-life. This process continues for any number of important or controversial issues. Individual voters seeking to align themselves with a party can compare each party's stance or platform and decide which one is most in line with their own personal beliefs.
Another purpose for these groups is strength in numbers. The United States is not a true democracy, but more of a representative republic. Instead of a logistical nightmare of 300 million individuals running for elected office every few years, different political parties actively seek out qualified candidates with the necessary skills and belief structure to represent their party's platform while serving their terms. Not all political candidates agree completely with their party's stance on individual issues, but it would be fair to assume a Democratic candidate would be of a moderate to liberal bent, while a Republican candidate would be moderate to conservative in his or her thinking.
The two major political parties have been accused of choking out the voices of other groups with far fewer resources. There have been third parties which have had noticeable effects on the outcome of national elections, however. Third party candidates such as Ross Perot and Ralph Nader have had the ability to draw votes away from major political party candidates and make voters aware of their parties' platforms. Many smaller parties, however, routinely take controversial stances on issues such as the legalization of marijuana, or put up fringe candidates as a grassroots political exercise.