In general, all "voltages" on a car will be 12 volts DC or "Direct Current".
A relay is just an electrically operated switch.
Most automotive relays are designed to make contact when power is applied. Like a horn relay. You press on the horn and that activates a relay, which in turn connects two switch contacts and allows the horn to sound.
Other relays turn off when power is applied.
And yet other relays have three switch connections. There is a "common" and then an off connection and an on connection.
The connections are called N.O. for normally open. And N.C. for normally closed.
A "two switch connection" relay would be called SPST for Single Pole Single Throw.
A "three switch connection" relay would be called SPDT for Single Pole Double Throw. (The kind you should get. One of the connections is on when power applied, the other connection is off when power is applied.)
In addition to the switch connections on a relay, there are two "coil" connections. These are the connections which operate the relay. For some automotive relays (also called solenoids in the automotive world), the - [minus] or ground connection is the metal mounting bracket which would be screwed to a metal body part and in turn be connected to the - terminal of the battery via that.
Note relays which are always on can get to be VERY hot. They also make "clicking" sounds when activated. For these reasons, automotive relays are typically mounted in the engine compartment of a car. They get good air circulation there and keep the clicking noise outside of the passenger compartment.
Also there are two different types of relays for automotive use. One is called "continuous duty". The other is called "intermittent duty". The first type is designed to be on for long periods of time - like for headlights. The other is designed to only be on for brief periods of time like for a horn. If you keep an intermittent duty relay on for long periods of time, it may overheat or burn out.
Then automotive parts generally don't have these "specifications" included in the packaging or anywhere else. They usually just have a part number! Other relays like for electronic or electrical use come with detailed specifications and have "data sheets" on the manufacturer's web site. Like this...
The other thing you need to be concerned with is the "load" or what you are connecting this to...
What are you connecting this to?
(This will determine the "current rating" or amperage the "contacts" will be able to handle.)
How relays work...