Transistor-transistor logic (usually referred to by its acronym, TTL) is a logic family which replaced the earlier diode-transistor logic. It is the circuit type inside the most widespread early integrated circuit logic family, the 74 series.
In TTL, gates produce low outputs by sinking current to ground (drawing it into the gate through the output, seemingly paradoxically): the collector of the gate's output transistor is connected to the gate's output, and the emitter to ground, and when the gate is on, for a low output, current flows through the transistor to ground. When the gate is off, the output is pulled up via a resistor connected to the supply voltage.
Use of a smaller pull-up resistor results in a gate which can switch faster, but draws more power; similarly a larger one produces a transistor that is slower, but uses less power. These approaches are used in the 74H and 74L TTL families, respectively.
One sub-set of TTL uses a pair of transistors in a so-called 'totem pole' output arrangement, with the collector of one attached to the supply voltage, the emitter of the other attached to ground, and the remaining emitter and collector are both connected to the output. The two transistors are then driven by the desired logic signal and its negation. This produced a gate which has symmetric high->low and low->high switching.
Another sub-set of TTL are open collector gates.
The earliest TTL chips are classed as SSI, but eventually TTL chips ranged up to the largest MSI. Building larger chips out of TTL was not feasible, as TTL logic tends to dissipate fairly large amounts of heat, which is what led to its eventual superceding by MOS.
Most computers from the late 1960s through to the middle 1970s were produced entirely in TTL (although its use in such ways continued until the early 1980s - e.g. the IBM 3081 was built out of TTL), and TTL continued to be widely used for the 'glue' logic between larger VLSI chips until the 1990s, when the lower voltages used with most CMOS made them obsolete.
- TTL article on Wikipedia - has comprehensive coverage of the topic