An SSL connection, if set up correctly, scrambles the data you post to a website and the data you get back. If you're reading or sending email, the SSL connection will hide your bits from prying eyes hiding in any of the computers or routers between you and the website. If you're going through a public Wi-Fi site, it makes sense to use SSL to stop the site or anyone using it from reading the bits you're sending back and forth.
SSL only protects the information as it travels between your computer and the distant website, but it doesn't control what the website does with it. If you're reading your email with your Web browser, the SSL encryption will block any router between your computer and the email website, but it won't stop anyone with access to the mail at the destination from reading it after it arrives. That's how your free Web email service can read your email to tailor the ads you'll see while protecting it from anyone else. The Web email service sees your email in the clear.
There are a number of complicated techniques for subverting SSL connections, such as poisoning the certificate authentication process, but most of them are beyond the average eavesdropper. If you're using a local coffee shop's Wi-Fi, SSL will probably stop the guy in the back room from reading what you're doing, but it may not block the most determined attacker.
Online privacy technique No. 4: Encrypted messages While Tor will hide your IP address and SSL will protect your bits from the prying eyes of network bots, only encrypted mail can protect your message until it arrives. The encryption algorithm scrambles the message, and it's bundled as a string of what looks like random characters. This package travels directly to the recipient, who should be the only one who has the password for decrypting it.
Encryption software is more complicated to use and far less straightforward than SSL. Both sides must be running compatible software, and both must be ready to create the right keys and share them. The technology is not too complicated, but it requires much more active work.
There's also a wide range in quality of encryption packages. Some are simpler to use, which often makes for more weaknesses, and only the best can resist a more determined adversary. Unfortunately, cryptography is a rapidly evolving discipline that requires a deep knowledge of mathematics. Understanding the domain and making a decision about security can require a doctorate and years of experience. Despite the problems and limitations, even the worst programs are often strong enough to resist the average eavesdropper -- like someone abusing the system admin's power to read email.