Some of you might have noticed a message at the top of your Gmail inbox, just below the search bar that says, “Coming Soon: Better Ads in Gmail.” This notification, which will slowly roll out to all Gmail users, signals Google‘s new ad system for its mail service; one that will target ads based on your interests within your email.

On the information page outlining the changes, Google states that bad ads tend to annoy people. True. Some people don’t even like good ads. Nevertheless, Google’s reasons for targeting ads based on your interests are limiting irrelevant ads and making the ads you do see much more useful.

Last week we told you about Facebook’s beta testing of real-time targeted ads. These ads would appear just after you updated your status or posted a comments on a friend’s wall. The Gmail ads will work in a similar way, in that the content in your email will provide clues for what ads would be best for you. For instance, if you and your friends frequently communicate about creating a softball team for the fall, you may see sporting goods ads within Gmail. On the flip side, Google points out that if you mark communications as spam and never read them, you most likely don’t want to see an ad related to the spam.

The news ads are set to work like Gmail’s Priority Inbox, which attempts to sort messages into categories based on importance:

With features like Priority Inbox, we’ve been working hard to help sort through the ‘bacn’ in your messages — the unimportant messages that get in your way. Soon we’re going to try a similar approach to ads: using some of the same signals that help predict which messages are likely to be important to you, Gmail will better predict which ads may be useful to you.

As this announcement is sure to ruffle some privacy feathers, Google makes sure to link to their Gmail ads policy. Here we are reminded that nobody is actually going to comb through our emails:

Ad targeting in Gmail is fully automated, and no humans read your email in order to target advertisements or related information. This type of automated scanning is how many email services, not just Gmail, provide features like spam filtering and spell checking. Ads are selected for relevance and served by Google computers using the same contextual advertising technology that powers Google’s AdSense program.

If that fails to assuage your trepidations, you will be able to opt out of this new advertising via the Gmail settings page. I’m not holding my breath for the Facebook targeted ads opt-out offer.