A political party is a group of people who come together with the aim of winning power.
The level of involvement from people can be as voters, supporters and members. It can also be as candidates, who if successful become elected representatives for their chosen political party.
Successful candidates belonging to a political party win a range of powerful positions on behalf of their parties. For example, to be a councillor for their local area, be a Mayor or become a Member of the UK Parliament.
A party can come together just for a local issue but usually a political party is a group of people who want to win governmental power.
Most political parties have natural supporters through their history and policies. For a party to win governmental power they also need to appeal to these natural supporters, but this is not enough to win.
Parties typically put their aims across in a written document called a manifesto.
They then ask people eligible to vote on a local, regional or national level to share their aims and vote for the candidates they put up at elections.
The Little Book of Politics: A Pocket Guide to Parties, Power and Participation by Stephen GaugeĀ (paid link)