UJT Negative Resistance and Ohm's Law
Ohm's law describes the proportionality of current to applied voltage in an ideal resistive material.

Even real passive parts do not behave in exact accordance (for example, a resistor may have a measurable voltage coefficient). 

Things like diodes are very nonlinear, so Ohm's law does not apply (though for very small changes in a known current it may be useful to apply it in the form of differential or dynamic resistance). The differential resistance of most types of diodes is positive, with some exceptions.

Differential resistance is just \$R_d=\frac{\Delta V}{\Delta I}\$ for small changes about a bias point. 

Negative (differential) resistance is similar- except the sign of the differential  resistance is negative. It should be obvious that this is only possible for a region of applied voltages, otherwise we could extract unlimited energy from the device.