Emitter-coupled logic (almost always used as an acronym, ECL) was, for several decades, the fastest logic family, reaching peak speeds of less than 1 nsec delay.
The basic concept in ECL is that in each gate's output stage, there are two paths, to two different outputs, with a constant current always flowing through one of the two. As the gate operates, the current is switched between one output and the other; the pair thus form complementary outputs.
The availability of complementary outputs has a number of advantages: individual inverters are almost never needed, reducing the total number of gate delays; the outputs can also be fed directly into a differential pair.
Since there is always a constant current flowing, ECL gates do not create spikes on the power supply line when they switch; however, they use a lot of power (constantly).
An ECL gate output uses a pair of transistors, with both their emitters connected to the low voltage in the system (often ground) - hence the name ECL. The collectors on both, which are the outputs, are connected to the supply (high) voltage through small resistors.
The base of one transistor is connected to a reference voltage, roughly half-way between the high and low supply voltages. The base of the other transistor is the input. When it is slightly above the reference voltage, all the current flows through that transistor; when slightly below, all the current flows through the other.