Diode–transistor logic (DTL) is a class of digital circuit

Digital circuit

Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...

s that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function (e.g., AND) is performed by a diode network and the amplifying function is performed by a transistor (contrast this with RTL and TTL).
The DTL circuit shown in the picture consists of three stages: an input diode logic

Diode logic

Diode logic or diode-resistor logic constructs Boolean logic gates from diodes acting as electrically operated switches. While diode logic has the advantage of simplicity, the lack of an amplifying stage in each gate limits its application...

 stage (D1, D2 and R1), an intermediate level shifting stage (R3, R4 and V-) and an output common-emitter switching-transistor stage (Q1 and R2).