Very important question. The answer is in the FAQ list under
What units does NEURON use for current, concentration, etc.?
You'll find a link to the FAQ list on the Documentation page http://www.neuron.yale.edu/neuron/docs
If you wonder why some currents are in mA/cm2 (milliamps/square centimeter) while others are in nA (nanoamps), it will be helpful to read about density mechanisms (also called distributed mechanisms) and point processes in chapter 5 of The NEURON Book; if you don't have the book, then read this
Hines, M.L. and Carnevale, N.T.
The NEURON simulation environment.
Neural Computation 9:1179-1209, 1997
which is available from a link at http://www.neuron.yale.edu/neuron/nrnpubs