Functions of political parties
Political parties select and present candidates for election, develop policy platforms/manifestos, mobilize and educate voters, and (once in power) form the government and take responsibility for governing -- or, if not in power, act as an organized opposition holding the government accountable.
Political parties vs pressure groups
A political party seeks to win elections and directly hold government power, standing candidates under its own name. A pressure group seeks to influence government policy on specific issues (like labour rights, environmental protection, or business interests) without contesting elections or seeking to govern itself.
Example 1: An organization campaigns for stronger environmental protection laws by lobbying lawmakers and organizing public awareness campaigns, but does not put forward candidates for election. Is this a political party or a pressure group?
This is a pressure group -- it seeks to influence government policy on a specific issue (environmental protection) through lobbying and advocacy, but does not attempt to win elections or hold government power directly, which is what would define it as a political party instead.
Party systems
A one-party system legally permits only a single party to hold power, common in some authoritarian states. A two-party system has two major parties that realistically compete for power (though smaller parties may exist), as traditionally seen in the USA. A multi-party system has several parties genuinely competing, often leading to coalition governments where no single party wins an outright majority -- common across much of Africa and Europe, including Nigeria.