Capacitors store electric charge and are used in timing circuits and to smooth DC power supplies. Capacitance measures a capacitor's ability to store charge. Resistors restrict electric current and resistance is measured in ohms, with resistor values often given in kilohms and megohms. Transistors amplify current and can be used as switches or amplifiers, with NPN and PNP types having different circuit symbols and compositions.

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Electronics Components
Capacitors
Capacitors store electric charge. They are used with resistors in timing circuits because it
takes time for a capacitor to fill with charge. They are used to smooth varying DC supplies by
acting as a reservoir of charge. They are also used in filter circuits because capacitors easily
pass AC (changing) signals but they block DC (constant) signals.
Capacitance is a measure of a capacitor’s ability to store charge. A large capacitance means
that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is
very large, so prefixes are used to show the smaller values.
Resistors
Resistors restrict the flow of electric current, for example a resistor is placed in series with a
light-emitting diode (LED) to limit the current passing through the LED.
Resistance is measured in ohms, the symbol for ohm is an omega Ω.
1 Ω is quite small so resistor values are often given in k Ω and MΩ.
1 k Ω = 1000 Ω 1 MΩ = 1000000 Ω.
Transistors
Transistors amplify current, for example they can be used to amplify the small output current
from a logic IC so that it can operate a lamp, relay or other high current device. In many
circuits a resistor is used to convert the changing current to a changing voltage, so the
transistor is being used to amplify voltage.
A transistor may be used as a switch (either fully on with maximum current, or fully off with
no current) and as an amplifier (always partly on).
There are two types of standard transistors, NPN and PNP, with different circuit symbols.
The letters refer to the layers of semiconductor material used to make the transistor. Most
transistors used today are NPN because this is the easiest type to make from silicon. If you
are new to electronics it is best to start by learning how to use NPN transistors.
The leads are labelled base (B), collector (C) and emitter (E).