There are times when you may need to boot to a volume other than the main OS X installation on your Mac, whether to try a specific software configuration without affecting your main workflow, or to troubleshoot an existing OS X installation by loading a fresh one from an external drive, or even to boot and use a Linux or Windows installation.
The standard way to do this for Apple's systems is very simple. When you boot your computer, hold down the Option key -- before or after hearing the boot chimes, depending on the type of keyboard you're using. Holding down the Option key will instruct the firmware to display available boot drives to load from.
For systems with hard-wired keyboards, such as USB keyboards or built-in ones, you can hold the Option key at any point after pressing the power button, up to when the boot chimes sound. However, for wireless keyboards the system will initialize the wireless controllers at the boot chimes, so key presses before this point will not be recognized and will fail to show the boot menu.
If you have a bootable DVD available, another option is to insert the disc and then hold the C key down, which will instruct the firmware to immediately boot from the optical drive instead of the main hard drive.
If you have enabled a firmware password on your Mac, then when booting to alternative volumes you will have to supply the password when prompted before the boot menu will show up. Otherwise, the system will simply boot to the main boot drive.
A third option for loading a secondary boot disk is to use Apple's Startup Disk system preferences. Like the boot menu, this system preferences pane will show any available boot device (optical, external, or internal). Selecting a boot device will set the system's firmware to boot to that device once the system is restarted.
Be aware that this will remain the default boot drive unless you change it again. Temporarily setting a different default may be beneficial if you need to load the selected OS and then repeatedly reboot to it, as it means you won't need to hold the Option key.