A TDR sends a pulse down a piece of co-ax and allows the reflectin to come back. Usually it is used either to measure teh cable (with a known velocity factor) or to measure the velocity factor (with a known length of cable).
This simple design creates an oscillator with a 74AC14 (fast rise time shmidt inverter). Using an oscilloscope on the output connector you can trigger on the outgoing pulse and watch for the return pulse. THe time between the rising edge of the outgoing pulse to the rising edge of the returned pulse is goverened by the equation:
Time = 2 * length_ of_co-ax / (Speed_of_light * Velocity_factor)
This is usually a figure in nS (for my test cable I had 25m of approx 0.81 velocity factor which lead to around 170nS between rising edges.