Most stereo and home theater receivers have a Speaker A/B switch for powering a separate pair of speakers in another room. A simple multiroom system can be setup by running speaker wires from the receiver's Speaker B terminals to two additional speakers in another room. This type of system is single-source - in other words, all zones hear the same source (CD, radio, etc.). It's an economical way to have music in two rooms using one system.
While most receivers have a Speaker B switch, it is possible to connect the receiver to more speakers if you install a speaker switcher with an output for each room. By adding a switcher, one system can provide music in any room, even the patio or garden. Read more about running speaker wires to other rooms.
It's a good idea to consider a separate volume control for each room. This model from On-Q controls volume and has impedance matching (meaning it protects the amplifier) and fits into a standard residential switch box.
As you can see, some projects are do-it-yourself and others may require professional assistance, such as running wires through walls. Learn more about finding a professional installer.