The term party system denotes the pattern of relationships that exist among parties in a political system. The type of party system which exists in a given state tends to be a product of such factors as the form of government that is in practice, what is prescribed in the constitution, the presence or absence of deep social cleavages, tradition, and so on. In democratic societies, the nature of the electoral system is also a prominent factor in the definition of the party system. The nature of the party system is important, as it has a tremendous impact on the manner in which parties behave.

In a one party system there is only one party which is permitted to exist by law. This single party controls every level of government and is primarily concerned with the tasks of mobilization, communication, and control. This tends to be the norm in non-democratic states. However, a stream of this, known as the one party dominant system, can exist in democratic states. In this system, other parties are permitted to exist, but one party regularly wins every election. For example, the Liberal Democratic Party governed in Japan for close to forty years until 1993 when allegations of party corruption led to its electoral defeat. These systems always have the possibility to evolve into a two party system because other parties are allowed to participate freely in the political system and provide alternatives to the governing party.

A two party system is dominated by two political parties, neither of which can ever count on maintaining power all the time. Although other parties exist, they tend to be minor in nature because these two main parties are usually the only ones who have a real chance at winning. This system provides a significant degree of choice between the two parties while simultaneously maintaining straightforward and stable governance, as one party generally secures a majority of votes. It tends to be characteristic of states with a plurality electoral system. Under this system, both parties must strive for a broad-based appeal.

Multi-party systems are characterized by the division of popular support among several political parties which tend to be quite distinctive. Thus, coalition government is a natural consequence in order for the government to operate efficiently. It tends to be the norm in an electoral system which functions on some form of proportional representation. Although this party system may offer the voter the greatest deal of choice, it is plagued by the inherent instability of coalition government.