A relay is basically a switch that is controlled by another electrical circuit rather than by moving a lever or pushing a button. Most common relays use a standard layout known as a "Bosch relay". They have four or five terminals with specific designations.


Bosch style relay terminals

Terminal 30 is connected to the power source you want to use with whatever device you're connecting to the relay (horns, fuel pump, etc.). It is often battery (constant) power but it could be ignition controlled or even controlled by a separate switch.

Terminals 87 and 87A are the output terminals that get connected to your device. They connect to terminal 30 internally and toggle depending on whether the relay coil (the part connected to the low current circuit) is energized or not. Terminal 87A is connected to 30 when the relay is at rest and is called the "normally closed" terminal. There is no 87A on a 4-pin relay. The more commonly used output is on terminal 87. It only gets connected to 30 when the relay is energized and is called the "normally open" terminal. Since relays are usually used to turn on devices, terminal 87 will be off when the relay is inactive and on when the relay is energized.

Terminals 85 and 86 are the relay coil connectors. One will be connected to power and the other to ground. It doesn't really matter which is which although common practice is to connect 86 to power and 85 to ground (this is because some relays have a quenching diode in them that helps extend terminal life and it needs that current direction to function properly). This is the low current circuit that controls the switching in the relay. When current flows through these two terminals, it magnetically moves the switch from 87A to 87, directing power from 30 out through 87.


Relay internal schematic

As an example, a fuel pump relay would have pin 30 connected to the battery, 87 connected to the fuel pump, 86 connected to an ignition switched power source and 85 to ground. When the ignition is switched on, power flows to 86, through the relay electromagnet to ground on 85. This energizes the electromagnet which moves the internal switch from 87A to 87 allowing current to flow from the battery through 30 to 87 and on to the fuel pump. When the ignition is switched off, the relay is no longer energized and the switch goes back to 87A cutting off power to the fuel pump through 87.

This is probably way more than you wanted to know about relays but it gives you an idea of how to connect them for any use you might have, regardless of what wire colors are connected.