OS X Mavericks: About speech recognition
With speech recognition, you control your Mac and apps by speaking commands that open apps, choose menu items, email contacts, and more. OS X provides many commands, or “speakable items,” to which you can add your own. Use the Speakable Items pane of Accessibility preferences to turn on and use speech recognition.
When speech recognition is on, a round speech feedback window with a microphone icon appears on your desktop. This window shows the key (such as Esc) to press or the keyword to say before you say a command. Click the triangle at the bottom of the window for quick access to preferences and the Speech Commands window, where you can see available commands.
If you have more than one microphone, speech recognition selects one, using this order: external USB microphone, external analog microphone, built-in microphone.