Our Constitution doesn't talk about political parties, but when our government was formed, some people with common interests got together to form groups to work for what they wanted from the government. Some wanted really strong government and others wanted less governmental control of the people. Each party, or group, picks candidates to run in national elections. When people register, or sign up, to vote, they can be a member of one of these parties - or they can choose not to pick a party and instead sign up as Independents. In most states, though, only voters who register with a party can vote in the state's primary elections.
Primaries are the elections in which parties choose their candidates for the general election.
The place where people vote is called a poll. The law says that polling places must be easy to get to. Schools, fire stations and courthouses are some places used for voting. Bars, or places where alcohol is served, cannot be polling places. Nowadays, you do have to go to a poll to vote and most people use a machine to cast a ballot.