Private browsing isn't a cloak of Web invisibility, and if you don't understand exactly what it does and doesn't do, you could find yourself pretty embarrassed.
It's an ongoing battle -- companies large and small do their best to track our Web browsing habits, while we do our best to keep our surfing as private as possible. That's why it was so huge when the major browsers started rolling out "private mode" browsing. Finally, we could click a mouse and become invisible…right? Not so fast. Private mode, or "porn mode" as it's sometimes derisively called, isn't really as private as you might think. Depending on what you want to use privacy features for, it may be just what you need, but it's not a cloak of Web invisibility, and if you don't understand exactly what it does and doesn't do, you could find yourself pretty embarrassed.
What does Private Mode do?
In a nutshell, Private Mode prevents your browser from saving information locally. In every major browser, private mode does two very cool things: clear cookies and wipe your history and temporary files. The former keeps sites, advertisers and other entities from tracking your browsing. The latter prevents, say, your partner from finding out that you just spent the last two hours looking for decorations for a surprise party. (Hey, you can use private mode for good as well as evil!)
On the other hand, if you can't hand off cookies, game sites might not track your scores and items you buy may not stay in your shopping cart for later purchase. Disabling your history means the next time you want to find a site, your address bar won't autocomplete the URL for you. If you've enabled saved passwords in your browser, that functionality won't apply to passwords you enter during a private session, so make sure to remember any passwords you create while incognito.
While you can individually control how your browser handles cookies and history, Private Mode lets you disable them on demand without having to change individual preferences.
Apple was the first to add a "private mode," called Private Browsing, back in 2005. Private Browsing is easy to find in the Safari menu. You'll be asked to confirm whether you really want to browse privately. Strangely, Safari doesn't ask you to confirm when you want to stop. While you browse privately, Safari will accept temporary cookies so that you can log into sites or make purchases, but when you disable Private Browsing those cookies are tossed.
In addition to clearing cookies and blocking history, Private Browsing also clears completed downloads from Safari's Downloads window, and any search terms and URLs you type disappear after you delete them from the Address Bar, so that the Autocomplete function doesn't come back to haunt you. Safari will also suspend syncing with iCloud for the duration of your session.
Microsoft calls its privacy mode "InPrivate Browsing," and also sweeps clean all traces of your browsing when you're done. Unlike Safari, which puts the entire browser into privacy mode, IE enables you to open private tabs while browsing normally in others. This opens up new uses for the feature, such as signing into two Gmail accounts at the same time.
To open an InPrivate tab in Windows 10 within the Windows UI, choose "New InPrivate Tab" from the tab bar "…" menu. Otherwise, choose "InPrivate Browsing" from the Tools menu.
If you are using Internet Explorer 9, right click on the Internet Explorer icon in the task bar and choose "Start InPrivate Browsing."
When you choose "New Incognito Window" from the Chrome menu, you'll enter the feature Google calls "incognito mode." Like other browsers, Chrome will accept cookies during your incognito session but delete them when you close the windows. It will also scrub your browser and search histories. Like IE, Chrome allows you to open both incognito and regular windows at the same time, so be careful that you don't inadvertently use a regular window for your "private" browsing.
Another gotcha you might encounter with Chrome is its connection to Google's Web properties: if you sign into your Google account from an incognito window, your subsequent searches will be saved in your online Google Web history. To completely cover your search tracks, you must disable Web history in your Google account.
Private Browsing in Firefox covers all the usual bases, leaving no cookies, cache or history when you're done. Firefox also adds the option to always start in Private Browsing mode in the browser's Preferences window. For per-session privacy, choose "Start Private Browsing" from the "Tools" menu.
What Doesn't Private Mode Do?
The largest flaw in Private Mode is that sites can still track your browsing using your computer's IP address. What's more, so can your Internet Service Provider and your employer's IT department. For that reason (among others), Private Mode browsing isn't for surfing forbidden sites while on the job, for example. Your own home's router can even betray you if you -- or someone else -- set it to log usage.
Private Mode usually doesn't block cookies entirely, so your web activities can be tracked during your private session. Most browsers also offer a "do not track" preference, although bear in mind that "do not track" works a lot like "do not call" -- it relies on other parties to respect the request. Bottom line: Don't trust that you are not being tracked.
If you've downloaded any files during your private session, those files will remain on your computer until you manually delete them. Same for any bookmarks you make while you're incognito. Older browser versions didn't scrub Flash cookies, so update to the latest version for the best privacy.
Finally, private mode won't protect you from malicious software that spies on your browsing habits, and as Google warns you when you open an incognito window, you'll still have to look out for people standing behind you.