difference between reverse saturation and leakage current
In an ideal diode, they are the same. The diode equation is 

$$I = I_s \left[\exp\left( \frac{qV}{n k_B T}\right) - 1\right]$$

so if you apply a strong reverse bias (\$v \ll 0\$), then the reverse current will be very close to \$I_s\$, the saturation current.

In a real diode, there may be other leakage paths aside from through the PN junction itself that allow current to pass, so the leakage current may be greater than the saturation current. For a diode mounted on a PCB, there could be additional leakage due to surface contamination.