Germany and America are both democratic republics, in that their governments are popularly elected, they have universal suffrage rights, and decision making is based on the democratic principles of majority rule and individual rights.
But the democratic systems each country is run on are different, and this affects which political parties exist, and how they influence the government.
In America, there is a two-party system, where Democrats and Republicans have broad platforms of policies intended to represent large swaths of the population. Money in the political system is concentrated in these two parties, so third parties are largely irrelevant, meaning political minorities have little to no effect on government policy.
In Germany, they have a parliamentary system where legislative seats are apportioned based on the percentage of the vote each party receives. This means there are multiple parties, and getting a majority of the vote is difficult. It requires the parties to make alliances of convenience in order to cobble together a government. So while a party might get only 5% of the vote, it might be all that is needed for a larger party to get the majority, select a Prime Minister and form a government. So in Germany, smaller parties are much more well represented both in Parliament and in policy decisions.