What is Resistor–transistor logic?

Resistor–transistor logic ] is a class of digital circuits built using resistors as the input network and bipolar junction transistors as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic and transistor–transistor logic. RTL circuits were first constructed with discrete components, but in 1961 it became the first digital logic family to be produced as a monolithic integrated circuit. RTL integrated circuits were used in the Apollo Guidance Computer, whose design was begun in 1961 and which first flew in 1966.