One of the more common devices we use are resistors. By the name you can probably deduce that their purpose is to resist the flow of current. Resistors come in a wide range of values, the unit of resistance being the Ohm and also a wide range of power ratings, the unit being the Watt. Both values are important when using resistors. Resistors values and ratings can usually be identified by coloured bands around them. If you are lucky though the value may be printed on as numerals instead. The resistor colour code may be remembered by the use of an extremely politically incorrect rhyme which will absolutely not be reproduced here for fear of litigation. Ask anyone working in the electronics industry, they will be able to tell you.
Another component which is most useful in model railway electrics is the diode. Put simply, this device conducts current in one direction but not in the other. There are of course many different types of diode. For our uses we really want ones rated at about two amps forward current and about thirty volts peak inverse voltage in the non conducting direction. A derivative of diodes that are most useful to us is the diode that lights up when passing current. They are of course called light emitting diodes. It is critical that they are operated at the current specified and are protected from excessive inverse voltages.
Capacitors charge up to the voltage connected across them and will retain this charge for a short time when this voltage is disconnected. For model railways electrics they are most commonly used in power supplies for solenoid operated point motors when a large amount of energy is required for a very short period. Larger electrolytic capacitors are polarity dependent and must be connected accordingly.
Relays can be considered as electrically operated switches, and are useful for controlling train routing and interlocking colour light signals.
As with all electrical devices or even any kind of machinery, it is imperative that you do not operate the devices beyond their stated tolerances. Failure to adhere to this advice would be stupid and dangerous and possibly result in personal injury, overheating and fire.