Hi,
See a photo of the relay:
The relay COIL is rated at 12VDC and when you apply 12V DC to the coil the electromagnet inside the relay switches the physical contacts.
The relay CONTACTS are rated as shown on the relay. One rating is 10 Amps (or LESS) at 30 VDC (or LESS) .
So you can switch 12V DC at up to 10 amps.
Reality Check: IF the load you are switching is inductive (Like motors, solenoids, other bigger relay coils) you need to protect the relay contacts from the dreaded
A DC spark / arc could continue for some time, acting like a miniature welder and welding relay contacts together.
For solutions see the article on the ArduinoInfo.Info WIKI HERE: (See the "Turning stuff ON and OFF).
Also, switching power loads can cause ElectroMotive Interference (EMI). See THIS about That...