Note that I’m using European breakers and EU-type panel design as my example, so if you’re in North America or certain other countries, you’re not gonna see the same thing. Still, the general principles are the same.
Also keep in mind that there are far more gizmos that you might find in your panel: combined GFCI/circuit breakers, timers, and even a nifty gizmo that turns off circuit B when circuit A is energized.
Having said all that, if you just need a basic intro to breaker panels of all types, this is a good place to start! Then follow up with a book or an internet search for your specific location, and Bob’s your uncle.
Power outlet tester:
And remember: Always turn the power OFF first!