A political party is an organization that sponsors candidates for political office under the organization's name.
True political parties select individuals to run for public office through a formal nomination process, which designates them as the parties' official candidates. Sponsoring candidates that are designated representatives is what defines an organization as a party. This distinguishes political parties from interest groups. Interest groups support political candidates but do not nominate them to run as their avowed representatives. If an interest group did nominate a candidate, then it would be by definition a political party.
Parties contribute to democratic government by functioning as part of the political system. The political system is the network of institution that links people with the government. The four most important party functions are:
Nominating candidates so that voters' choices are quality controlled and not just a slew of self-nominated candidates. Party insiders judge a candidate's suitability for representation of the party, and sometimes even recruit candidates of high quality.
Structuring the vote choice by reducing the number of candidates on the ballot with a realistic chance of winning. Established parties have experience in contesting elections and acquire a following of loyal voters who guarantee the party's candidates a predictable base of votes. This discourages non-party candidates from running for office and new parties from forming, thus minimizing the amount of information a voter needs to make a rational choice.
Proposing alternative government programs with general policies that the candidates pledge to pursue if elected. Candidates of the same party usually favor policies that fit their party's underlying political philosophy. Thus, a voter familiar with a party's political ideology can make assumptions about that party's candidate.
Coordinating the actions of government officials -- political parties are the major means for bridging the separate powers (president, House, Senate, et al) to produce coordinated and efficient policies. This is performed via: candidates' and officeholders' political fortunes are linked to their party, which an bestow and withhold favors; members of the same party in the presidency, House and Senate often voluntarily cooperate in making policy.
However, the Constitution makes no mention of political parties and none existed when it was authored in 1787. At the time, groups pursing common political interests were known as factions. Factions, though naturally occurring, were thought of as dangerous. Federalist No. 10 proposed that the Constitution's federalism would prevent factions from controlling the government.
Regardless, factions existed even under British rule. Colonial assemblies referred to Crown supporters as Tories or Loyalists, while Crown opponents were known as Whigs or Patriots. After independence, the Constitution spurred federalist supporters and antifederalist opposers. However, these groups were not parties because they did not sponsor candidates for election.
Political parties are an organized attempt to gain control of the government through elections, according to Schattschneider's view. This is the most important feature distinguishing political parties from interest groups: interest groups want to affect specific policies -- Sierra Club, NRA, et al -- while political parties want to take over the branches of government.
Political parties' primary objective is to win elections. Their secondary objective is to implement policies that: get them reelected; serve the national interest; bring about a good society. Note that political parties differ in their interpretation of an ideal society.
There is no Article of the United States Constitution that mentions the role of political parties. In fact, the Constitution was intended to prevent the emergence of a majority faction. Political parties make possible precisely the coordination that the founders feared (Madison in particular).
While not Constitutionally defined, political parties coordinate, take over and manage to overcome Constitutional barriers created by Madison and others. Madison wanted power distributed horizontally at the federal level by separation of powers and checks and balances, then disbursed vertically by reserving power to the states in the 10th Amendment.