1st Part of the question: The main political parties in the United States are the Democratic Party and the Republican Party (Also known as the Grand Old Party/GOP)
In modern America these are the two most powerful parties. The third most popular party is the Libertarian Party, though it does not hold any seats in the Congress.
2nd Part of the Question: I personally believe in an open free world with fair trade and opportunities for everyone. In that case, I knew Libertarian Party echoed my views but I also knew that the party can never ever win an election or any seats. It's just too small.
I would then usually for the Republican Party, but the latest candidate always said things which were completely opposite to what I believe in.
Hence, this time I voted for the Democratic party so I guess the candidates did affect my choice.