EU nationals do not require a visa to enter the Federal Republic of Germany.
Non-EU nationals
Most other foreigners do require a visa for stays in Germany. For visits up to 90 days in a 180-day period a visa is not required for nationals of those countries for which the EU has abolished the visa requirements. You will find a good overview on visa requirements on the website of Germany's Federal Foreign Office.
The Federal Foreign Office also provides detailed information on The Schengen Agreement and the Schengen Visa Regulations.