This is an NOR gate implemented using transistor-transistor logic. Click on the inputs on the bottom to toggle their state. When one of the inputs is high, the output is low; otherwise, the output is high.
When one of the inputs is low, the easiest path to ground through the corresponding 4.7k resistor is through the base of the transistor below it and to the input. This brings the transistor's collector voltage low enough so that very little current can flow through the base of the transistor on the right. This keeps that transistor off. If both inputs are low, both transistors connected to the output are off, and the output stays at 5 V.
When one of the inputs is high, the transistor to the right of it is in reverse active state. A current flows through the 4.7k resistor through the base and collector of these transistors, and then through the base of the transistor on the right, saturating it and bringing the output down near ground.
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