There have been several series of TTL logic:
7400: 10 ns propagation time, 10 mW/gate power consumption, obsolete;
74L00: Low power: higher resistances, less dissipation (1 mW), longer propagation time (30 ns);
74H00: High power: lower resistances, more dissipation: less sensitivity for noise;
74S00: Schottky-clamped: faster switching (3 ns, 19 mW) by using Schottky diodes to prevent the transistors from saturation;
74LS00: Low power, Schottky-clamped (10 ns, 2 mW);
74AS00: Advanced Schottky: faster switching, less dissipation, (1.5 ns, 10 mW);
74ALS00: Advanced Low power Schottky (4 ns, 1.3 mW).