Bases go to control circuit.
Its NPN and PNP examples.
Motor go forward T1 = 1, T2 = 0, T3 = 0, T4 = 1
Motor go Backwards T1 = 0, T2 = 1, T3 = 1, T4 = 0
Motor idling T1 = 0, T2 = 0, T3 = 0, T4 = 0
Motor brake (quick slowdown) T1 = 1, T2 = 0, T3 = 1, T4 = 0
Brake T1 = 0, T2 = 1, T3 = 0, T4 = 1
Avoid T1 = 1, T2 = 1 this would be bad, the same is for T3 = 1, T4 = 1. It would cause both transistors to come on giving a direct path to ground. The transistors would burn up, and you could damage your power source.
T1 and T2 are on left side, T3 and T4 are on right side.
T1 and T3 are on top side, T2 and T4 are bottom side.