Same pinout, but the gain is different. A 12AU7 only has a gain of 17 or so, a 12AT7 has a gain of 60. The 12AT7 was originally designed as a VHF oscillator or mixer tube, but it can be used in audio circuits.
Inherent Difference - 12AU7 and a 12AT7
Can anyone school me on the practical differences between a 12AU7 and a 12AT7 input tube when used in a preamp? For example, all things equal is one of these tubes quieter than the other on average, does one have greater gain than the other, and is there any characteristic 'sound' to each of these tubes?