The difference between instantaneous power and average power. The distinction is important because we use AC power.

You can see in the above that in AC power, the voltage is continuously varying so that one can define a Peak Voltage and an Average Voltage. The average voltage is usually called the root mean square (rms voltage)

V_rms = V_peak/1.414

Current, I, varies in a similar way as the Voltage such that power curve for AC would look like this:

Now let's consider this situation:

Peak Current = V/R = 34/8 =4.25
Peak Power = V x I = 34 x 4.25 = 144.5
V_rms = 34/1.414 = 24V
I_rms = 4.25/1.414 = 3A
P_rms = VxI = 24x3 = 72 Watts