Unit is Ohms, Symbol is R or Ω . Resistance is a measure of how easily (or with what difficulty) electrons will flow through the device.  Copper wire has a very low resistance, so a small voltage will allow a large current to flow.  Likewise, the plastic insulation has a very high resistance, and prevents current from flowing from one wire to those adjacent.  Resistors have a defined resistance, so the current can be calculated for any voltage.  Resistance in passive devices is always positive (i.e. > 0)