Quiet will ask programs which normally vie for your attention to hold their peace. Chat clients such as Adium or iChat by will be set as 'away' or 'busy'. Ringing in Skype will be turned off using the 'Do Not Disturb' mode. Quiet can even hide the Dock to prevent unwanted icon bouncing or badges reminding you of how many emails you have unread.
At the same time Quiet will hide unrelated windows, including the Finder itself, making the window or windows you wish to focus on the only thing on your screen.
Preserve your train of thought and be effective with Quiet.