Contactors and relays are terms that people find confusing. Some people think that they are simply two different ways of referring to the same component.  A component which is essentially an electrical switch.
First of all, contactors and relays perform the same function and share the same high level design. Both these components are used to provide current to electrical loads and they are used to control current that courses through an electrical circuit.
But a contactor is different from a relay and it could be dangerous to use one kind of electrical switch when you should be using the other as you will learn in this article.
Before we focus on the differences though, let’s look at the similarities.

Contactors and relays both have a top housing and a bottom housing.
The top housing contains the contacts. An electrical contact is typically a pair of metal conductors. When they touch they pass an electrical current. When they separate the current is turned off.
Contacts that touch refer to a closed switch. Separated contacts mean an open switch. Open switches allow the current to pass and power the load.
Inside the bottom housing of the contactor and the relay is the solenoid. The solenoid is a soft metal core with a coil of wire wrapped around it. When an electric current passes through the coil, the whole assembly becomes an electromagnet. An electrical field is generated that moves the contact in the top housing causing the electric current to flow or, when the electromagnet is de-energized it stops the flow of current by separating the contacts.
This is pretty much the basic and common functionality of a contactor and a relay.
Now let’s take a look at what makes these electrical switches different.

Looking at the differences between a contactor and a relay will also inform us about when a contactor should be used and when a relay should be used.

Load Capacity

One main difference between a contactor and a relay is the current that they are designed to handle. Contactors are designed for higher currents while relays are for lower electric charges.
The unit of measure for electric current is the ampere. 10A means ten amperes. Incidentally, 10A is also a good demarcation point for contactors and relays. Use relays when dealing with currents 10A or less. More than that, use contactors.
Another measure of electricity is the volt. A voltage is an electrical force or pressure that drives an electric current between two points. Relays are rated at 250 volts or 250V. If you’re dealing with more than 250V use a contactor.
The higher load capacity of contactors also make them both bigger and heavier than relays.

Network Functionality

Looking at the contacts of a relay they are either in the Normally Open (NO) or Normally Closed (NC) position. The contacts of a contractor are always in an open position also known as Form A.
This points to how they are typically used in a circuit.
Contactors are meant to open and close the switch multiple times in a circuit depending on the specific requirements of the network.
Relays on the other hand are meant to respond to changes in the surrounding circuit by either opening or closing the contacts.
The distinction is a bit subtle but contactors and relays do occupy unique niches in an electrical system.

Now it’s time to take a closer look at contactors. Because of their higher load capacity and slightly different functionality they have some features that further set them apart from relays.

AC and DC Contactors

There are two types of contactors: DC contactors and AC contactors.
Very simply DC contactors should be used for direct current (DC) loads and AC contactors should be used for alternating current (AC) loads.
It is important to note that they are not replaceable to each other.
AC and DC contactors have differences with each other because of the type of current they handle. The iron cores of each are made of different material and are shaped differently, for example.

Auxiliary Contacts

Aside from the main contacts in the top housing, contactors also have auxiliary contacts.
These  auxiliary contacts function apart from the main contacts.
While the main contact serves the primary purpose of the contactor, the auxiliary contacts may perform another, perhaps related, sub-function that typically will use less power.
For example, while the main contact brings current to an electric motor the auxiliary contact may power an indicator light that signals the electric motor is on.

Spring Loaded Contacts

Spring loaded contacts are used by contactors for greater stability and as added insurance that the current is turned off when the electromagnet is de-energized.

Arc Suppression

Remember when we were talking about the two different types of contactors? Arc suppression is mainly an issue for DC contactors. The alternating current that goes through an AC contactor has a high zero voltage occurrence. Electric arcs are extinguished when zero voltages occur. The direct current that goes through DC contactors never zeroes out so arc suppression technology is an essential feature of DC contactors.
An electric arc or arc discharge is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current going through the air produces plasma which, in turn, produces visible light. Electrical arcs can cause damage to the contacts inside the contactor housing , therefore DC contactors employ arc suppression. Arc suppression is the reduction of sparks formed when current carrying contacts are separated.
One such arc suppression technology are arc chutes. These are a set of metal plates that are insulated from each other and arranged in parallel.  They divide and extinguish electrical arcs.
DC contactors also adopt gas-based arc suppression technologies such as having ceramic housings filled with hydrogen or epoxy-sealed housings filled with nitrogen.  These gases create environments that dampen electrical arcs.

By now it should be clear that contactors and relays are two very different components and choosing one from the other depends on how much load capacity your system will be generating. Use contactors for more than 10A or 250V.
If you are going to use contactors you have to decide based on whether your circuit uses DC or AC current. DC contactors for DC loads and AC contactors for AC loads.