DEFINITIONS

Passive:	Capable of operating without an external power source.
	Typical passive components are resistors, capacitors, inductors and diodes (although the latter are a special case).
Active:	Requiring a source of power to operate.
	Includes transistors (all types), integrated circuits (all types), TRIACs, SCRs, LEDs, etc.
DC:	Direct Current
	The electrons flow in one direction only.  Current flow is from negative to positive, although it is often more convenient to think of it as from positive to negative.  This is sometimes referred to as "conventional" current as opposed to electron flow.
AC:	Alternating Current
	The electrons flow in both directions in a cyclic manner - first one way, then the other.  The rate of change of direction determines the frequency, measured in Hertz (cycles per second).
Frequency:	Unit is Hertz, Symbol is Hz, old symbol was cps (cycles per second)
	A complete cycle is completed when the AC signal has gone from zero volts to one extreme, back through zero volts to the opposite extreme, and returned to zero.  The accepted audio range is from 20Hz to 20,000Hz.  The number of times the signal completes a complete cycle in one second is the frequency.
Voltage:	Unit is Volts,  Symbol is V or U, old symbol was E
	Voltage is the "pressure" of electricity, or "electromotive force" (hence the old term E).  A 9V battery has a voltage of 9V DC, and may be positive or negative depending on the terminal that is used as the reference.  The mains has a voltage of 220, 240 or 110V depending where you live - this is AC, and alternates between positive and negative values.  Voltage is also commonly measured in millivolts (mV), and 1,000 mV is 1V.  Microvolts (uV) and nanovolts (nV) are also used.
Current:	Unit is Amperes (Amps), Symbol is I
	Current is the flow of electricity (electrons).  No current flows between the terminals of a battery or other voltage supply unless a load is connected.  The magnitude of the current is determined by the available voltage, and the resistance (or impedance) of the load and the power source.  Current can be AC or DC, positive or negative, depending upon the reference.  For electronics, current may also be measured in mA (milliamps) - 1,000 mA is 1A.  Nanoamps (nA) are also used in some cases.
Resistance:	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)
Capacitance:	Unit is Farads, Symbol is C
	Capacitance is a measure of stored charge.  Unlike a battery, a capacitor stores a charge electrostatically rather than chemically, and reacts much faster.  A capacitor passes AC, but will not pass DC (at least for all practical purposes).  The reactance or AC resistance (called impedance) of a capacitor depends on its value and the frequency of the AC signal.  Capacitance is always a positive value.
Inductance:	Unit is Henrys, Symbol is H or L (depending on context)
	Inductance occurs in any piece of conducting material, but is wound into a coil to be useful.  An inductor stores a charge magnetically, and presents a low impedance to DC (theoretically zero), and a higher impedance to AC dependent on the value of inductance and the frequency.  In this respect it is the electrical opposite of a capacitor.  Inductance is always a positive value.  The symbol "Hy" is sometimes used in (guess where :-) ... the US.  There is no such symbol.
Impedance:	Unit is Ohms, Symbol is Ω or Z
	Unlike resistance, impedance is a frequency dependent value, and is specified for AC signals.  Impedance is made up of a combination of resistance, capacitance, and/ or inductance.  In many cases, impedance and resistance are the same (a resistor for example).  Impedance is most commonly positive (like resistance), but can be negative with some components or circuit arrangements.
Decibels:	Unit is Bel, but because this is large, deci-Bels (1/10th Bel) are used),  Symbol is dB