This topic is all about visas – but not residence permits. Please understand as "visa" the permission to be inside Germany as a visitor for up to three months in a row. A "residence permit" allows persons to indefinitely live here and maybe also pursue some kind of employment and in the long run be eligible for permanent residency – longest after five years.

Denial of a Visa
Entry Visa
Extension
Formalities for Visas
List of Countries (not) Requiring a Visa to Enter Germany
Visa Application
Visa or not?
Visas
Visas Required for Countries Starting A
Visas Required for Countries Starting B