Wikipedia
Gastarbeiter (plural, "Gastarbeiter") is German for "guest worker" (literal translation). It refers to foreign or migrant workers, particularly those who had moved to West Germany (BRD) mainly in the 1960s and 1970s, seeking work as part of a formal guest worker programme (Gastarbeiterprogramm). Similarly, the Netherlands and Belgium had a parallel scheme called the gastarbeider programme. Sweden, Denmark, Norway and Finland had similar programmes called arbetskraftsinvandring (workforce-immigration).
East Germany (DDR) had a similar programme and referred to the workers as "Vertragsarbeiter".
These programmes mainly came about as a result of the severe labour shortage in continental Northern Europe following World War II and political pressure from southern European countries, Turkey and the USA.