Political partydemocracy, leaders must "run for office" in an election. In a dictatorship the party or leaders can approve a new leader without the people having a choice.
Parties are sometimes against the law - or only one party is allowed. New parties can come from social unrest and create a revolution. After they get power, they may make other parties illegal. For example, the Bolsheviks did this in Russia.
One way to say what a party is to say it is a power network devoted to politics. A party can help people from different social networks to do things. It means they can do things even if other people try to stop them. The social network does not share a community for this to happen but they have to agree on how to run communities.
One way to tell if a political party is doing a good job is to see if they care about the political virtues. Because politics is mostly about controlling war and violence, distraction can be quite important. Many people dislike political parties for paying too much attention to distractions and trying to maike themselves look better than other parties. This always happens because they are competing with the other parties.