What is the major difference between one-party and two-party systems?

That there is only one party ... A one party system is where there is one group of political views, this is how it was in nazi germany and in the soviet union when it was a communist nation. A two party system is when there are, like the name implies, two parties. The unites states is a two party system, republicans and democrats. There are also smaller parties such as the green party and libertarian party but it is mainly just the two.