Transistor–transistor logic is the circuit type inside the most widespread early integrated circuit family; it replaced the earlier diode–transistor logic. The name is derived from the fact that both logic as well as signal-amplification were performed by transistors.
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.
One extremely popular line of TTL chips was the 74xx series, produced by Texas Instruments; many other manufacturers eventually produced number-compatible TTL chips of their own (so that, say, a 7400 from Motorola was a pin-compatible replacement for the TI chip).
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 CMOS made them obsolete.