There are so many different types of relays, that it would be literally impossible for me to add them in this article. Therefore, i will categorize the types of the relays in terms of:

An octal 3PDT relay and it's base

There are basically two types of relays in this category. The first type is the normal on/off relay. This relay changes state as long as the electromagnet is actuated, and goes back into relax state when the electromagnet is not actuated any more. This is the most common relay type and is used widely in automation.

This type of relay operates just like a toggle flip flop. When the coil is once actuated, the relay will change state, and will remain in this state even if the coil in no more actuated. It will only change state again on the next pulse that will actuate the coil. This is very handy in modern house lighting. Having this relay instead of a switch, you can turn on and off the lights with one pushbutton. You press the pushbutton once and the lights are turned on. On the next pushbutton press, the lights are turned off.

A latching relay

This type of relay operates exactly like the R-S flip flop. It has two different coils instead of one. When the first coil is actuated, the relay goes to the SET position and it remains there no matter if this coil is kept actuated. It will only change state (to RESET position) only if the other coil is actuated. This type of relay is widely used in applications where the state of the relay needs to be kept as is, even after a power failure or a restart.

I will distinguish this type of relays in two sub-types. The first sub-type is the current-leaking protective relay, and the other type is the overload protection relay.

A current leaking protection relay

Almost everyone knows these relays. They does not actually have an electromagnetic coil. Instead, they remain armed all the time. Two electromagnets are placed one opposite the other. Between them, there is the armature. This armature is magnetized from both electromagnets. The first electromagnet is placed in series with the Phase, while the other is connected in series with the Neutral. If the current that flows through both electromagnets is equal, then the armature is kept in balance. But if the current that flows through the second electromagnet is less than the current that flows through the first electromagnet, then the armature is pulled to the first electromagnet that has greater magnetic force! And how can this happen? Easy, if somehow an amount of current flows to the ground of the installation...

These relays can (and SHOULD) be found in every household electrical installation, right after the main switch. Look at the following illustration:

The light bulb is turned on because the magnetic power from both coils is equal. Now look what happens if "somehow", the current on the neutral is less than the current on the phase. The magnetic power of the electromagnets is not equal, thus the relay will cut the power supply and our friend will be saved. For safety reasons, if this happens, the relay can only be restored mechanically, if someone pulls the lever of the relay up again:

An overload protection relay

Very common relays in motor applications, as well as in all electrical installations. These relays wave no electromagnetic coil to move the armature. Instead, they have a bimetallic strip that the current flows within. The material and the thickness of this strip is carefully selected, so that it will be heated (and thus bended) above a specified current value. When the bimetallic strip is bended, the relay will cut-off the power supply. For security reasons, the relay can only be restored mechanically by moving the lever by hand.

This is the basic idea of the overload protection relay.	If one line is overloaded, the bimetallic strip is overheated and thus it bends, breaking this way the contact

It should also be mentioned that there is another kind of overload protection relays called "electromagnetic relay". This operates exactly the same as the overload protection relay, but has inside also another electromagnet. If this electromagnet is powered, then the relay will be forced to break connection, as if it was overheated. This functionality allow to check for faults and stop a motor to avoid any other problem, even if the motor itself is not overheated.

A temperature relay, AKA thermostat

These relays operates similarly to the overload protection relays above. The major difference is that the bi-metallic strip is not heated by the current that flows within the strip, but from an external factor. This factor could be the ambient air, water temperature, another fluid refrigerator temperature etc. You may know these relays with another name... thermostats, used extensively in heating applications.

Another difference from the protection relays is that the temperature relays usually do not need an external mechanical movement to restore it's state. The process is done automatically according to the temperature of the bimetallic strip.

You could imagine a reed relay like a relay without an electromagnet. The reed relay's armature is actuated from any other external magnetic field. The reed relays can be found in door monitoring systems. A permanent magnet is attached to a door, while the reed relay is right above the magnet. If the door opens, the state of the reed relay is changed. Another common application for reed relays is on the speed meters of the bicycles. A permanent magnet is attached to the wheel of the bicycle, while the reed relay is fixed on the "fork" of the bike. Every time the wheel rotates and the magnet passes in front of the reed relay, it sends a pulse to a microcontroller.

There are many other types of relays like the timers and the function relays, but they use some kind of circuitry to perform different actions. I will not go into these categories, as this article is only interested to the kind of relays that uses no other circuitry, only mechanical variations.

Another type of relay categorization is the coil. In this category i separate the relays according to the way that their coil is powered to actuate the armature. So we have:

A permanent magnet is attached to the armature

This is a variation of the neutral relays. These kind of relays have exactly the same coil as the neutral relays, but they carry a permanent magnet on the armature. The polarization of the magnetic field of the coil depends on the polarity of the supply. Therefore, the armature is actuated only if the polarity of the coils' magnetic field is opposite to the polarity of the permanent magnet's magnetic field. This way, the relay is actuated only if the coil is correctly biased.

This kind of relays operates exactly the same as the biased relays. The only difference is that these relays does not have the permanent magnet, instead they have a diode in series to the coil. If the diode is correctly biased, the coil will have power and the armature will be actuated. The difference that makes these two relay types different is that the biased relays will allow the current to flow through it's coil, even it the relay is reverse-biased! Very important if someone wants to connect the coils of two or more relays in series.

This is the modern type of relays. These relays does not have a coil, nor any other moving part, that's why they are called Solid State. They are used for fast switching (up to several hundreds of Hz) and for controlling loads in explosive or harsh environments. They have significantly more lifetime than the conventional relays, as their contacts will not corrode due to humidity, dust or other causes. Actually, they do not have contacts! Instead, a FET or a TRIAC is used to simulate the contacts. The major disadvantage is the price...

As far as the contact number is concerned, the relays (like the switches) comes with some kind of coding. The general code form is this:

xPyT

The 'P' stands for "POLES". The 'x' is the number of "POLES" that a relay has. Thus, if a relay has 1 contact pair (POLE), the code would be SP as for Single Pole. For two contact pairs, it would be DP as for Double Pole. Above 2 contact pairs, the x gets the number of poles, eg for 3 poles it would be 3P etc etc.

The 'T' stands for "THROW" and 'y' is the number of "THROWS". 'y' can be Single or Double. Single Throw (ST) means that there is only one NO or NC contact. Double Throw (DT) means that the relay has pairs of NO/NC contact.

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