When you search on Google Desktop, you have the option to view your personal search results with (or without) Google.com search results. This feature is called Google Integration. Your personal results aren't actually visible to others on Google.com; they're only visible on your computer.
If you'd prefer not to see your personal results displayed with web search results, you can turn this feature off.
When you enter a search term, Google Desktop sends your query to two different programs and locations. One query goes to Google Web Search, and a duplicate query goes to the Google Desktop application running on your computer to search your personal index. Google Desktop intercepts Google Web Search's search results page, and adds your Google Desktop results above the web search results. This way, you see both results at once.