A political party is made up of people who have somewhat similar ideas on what needs to be done. They work together to try to get their candidate elected to a government office, and once their candidates are in office, they work together to pass the laws they believe are important. You don't need a party behind you to get elected, but it certainly makes it a great deal easier! The purpose of any political party is to win elections and influence the decision of government.
In the United States today, there are two strong political parties - the Democratic party (symbol: Donkey) and the Republican party (symbol: Elephant). Besides those, there are other parties, but they don't have as many members. People in the United States are invited to join a party. Many do. But some people do not choose to join a party. Those people are called Independents. Independents vote, but nobody knows which party will get their vote. Even though you belong to a party, you do not need to vote for your party's candidate(s) on election day. Every voter can vote for whomever they wish. A party works hard to get your vote.
Before a presidental election, each political party has a convention. The people who have been selected to represent their state at that convention have two jobs - (1) to select a candidate, and (2) to write up a platform, a list of things the party will support and work towards. For example, a political party might decide that they are going to try and implement a program that provides free health care for every American. That would be a wonderful thing. But such an idea would not happen unless a way could be found to pay for it. If the only way to pay for it is to increase taxes, the idea might be popular but the way to pay for it would not. Parties have to be very careful to create a platform that is affordable, necessary, and popular, so that their candidate will be elected. Once in office, people from a political party can team up with other elected officials from the same party, and vote the same way on things, to get things done the way their party believes things should be done.
Political parties get the word out about their candidate and platform with debates, slogans, speeches, fundraisers, advertising, email and letter writing campaigns, posters, signs in yards, and any other way they can think of to get their candidate selected by the voters.
See Also: Elections