Our government is not allowed to open your mail. It cannot listen in on your personal phone calls (see below), track your web browsing or your e-mails.
It can't search your home or office and definitely can not probe your body cavities. Here's the deal.
Authorities may not search anywhere you have a reasonable expectation of privacy unless there's probable cause to believe you're involved in a crime. Even then they have to get a warrant based on sworn statements in front of a judge. It's all part of the Fourth Amendment.
If the police conduct an illegal search, the evidence usually can't be used in court. Sometimes bad guys are set free as a result.
Not everything's off limits. The Supreme Court has ruled it's okay to go through your garbage. Listening to cell phone conversations is allowed because they're so easy to intercept they are not really private. Bag searches at airports, drunk-driving checkpoints and student drug testing - all permissible. You may have to open the trunk of your car if police think you're acting hinky.
When you hear people say "Big Brother is watching," they mean a totalitarian government spying on every part of our lives. Comes from an awesome book named "1984." Good creepy read.