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:
http://www.auswaertiges-amt.de/EN/EinreiseUndAufenthalt/03_Visabestimmungen/StaatenlisteVisumpflicht_node.html 

The Federal Foreign Office also provides detailed information on The Schengen Agreement and the Schengen Visa Regulations:
http://www.auswaertiges-amt.de/EN/EinreiseUndAufenthalt/03_Visabestimmungen/Schengen_node.html

Upon your request, the International Office will issue an offcial letter of invitation as part of your visa application.