If you're like almost every single Internet user on the planet, you probably use Google for something. Maybe Gmail, maybe Google search, maybe Google Docs, maybe Google Voice... or maybe all of the above.
Anytime you perform a Google search, for example, it's logged. Your computer's IP address and cookie (unique identifiers that can essentially pinpoint you and your location) are also included, so your computer's entire search history is archived.
When you receive an email through Gmail, or a voicemail on the Google Voice service, it's archived on their servers. Even if you delete the messages, there's still a copy on Google's servers. The marginal cost of digital storage is so ridiculously cheap that they have little reason to delete this data.
Then, of course, there are all the government requests for user data. In the first half of 2011, the US government requested information on over 11,000 Google accounts. Google complied with a full 93% of those requests. Your account might have been one of them, and you would never know.
It's not just Google either. Between Google, Microsoft, Yahoo, and AOL, the four companies power the email accounts of over 1 billion people. And all of them are in bed with the US government.
In a way, they have to be. They're all US companies-- headquartered in the US and subject to US law. When the government comes looking for information, or some judge decrees that a user's emails be confiscated as evidence, they have to comply.
Big Brother compliance also goes far beyond email. Skype, the popular instant message and VOIP software that was once thought to be private and secure, is now owned by Microsoft... meaning that Skype chats are now also subject to courts and police agencies.
So what to do? First, and most importantly, be mindful about what you put in an email or online chat platform. The best rule of thumb is that sending an unsecure email is like shouting the contents across the street.
Further, consider using an offshore email provider that can securely host your account abroad. There are a number of them available.