Essentially, your photos, messages and other documents are automatically encrypted when you set up a passcode, with or without a fingerprint ID to unlock the phone. Apple says it cannot bypass that passcode, even if law enforcement asks.
Apple, Google and other tech companies have been trying to depict themselves as trustworthy stewards of personal information following revelations that the National Security Agency has been snooping on emails and other communications as part of an effort to identify terrorists. Apple is also trying to reassure customers about its commitment to security and privacy after hackers broke into online accounts of celebrities who had personal photos stored on Apple's iCloud service.
Beyond setting up passcodes, some phones have additional tools for hiding or securing sensitive photos and documents stored on the phone, particularly if you need to lend or show your phone to someone.
Here's a closer look at some of those options:
Apple's iPhone and iPad
In the latest software update for mobile devices, iOS 8, Apple offers an easier way to hide photos from your collection in the Photos app. Simply press down on the photo or the thumbnail of it and tap "Hide."
However, the photo will still appear in individual albums, including a new one called "Hidden." You can go there to unhide hidden photos.
So why bother? This feature is mainly useful when you want to let people glance through your entire collection of photos. That could be when you're sitting with a friend in the same room or making a presentation before a large audience. You can hide embarrassing or incriminating photos - such as naked selfies - as long as you remain in control of the device. If you hand it to a friend and walk out, your friend can browse through the albums section.
You then go through your phone to mark certain content as private. With photos, for instance, just go to the Gallery app and select the photos or albums you want to keep private. Then hit the menu icon for the option to "Move to Private." This also works with selected video, music, audio recordings.
After you've marked your files as private, you need to go back to the settings to turn Private Mode off. Think of that setting as the door to a vault. Turning it on opens the door and lets you move stuff in and out. Turning it off closes and locks the door. It's the opposite of what you might think: Private Mode needs to be off for your content to be secure.
Once locked, it is as though the content never existed. No one will know what's inside the vault, or whether there's even anything inside. To unlock the vault, you need your passcode or fingerprint ID.
The private-mode feature is also part of Samsung's Galaxy Tab S tablets and the upcoming Galaxy Note phones.
LG's flagship phone has a guest mode. You can lend a phone to a friend without giving your friend access to everything. You can even set a separate unlock code for the guest, so that you don't have to give out yours.
Look for "Guest mode" in the settings under the General tab. You then specify which apps your guest can access. For instance, you might want to give access to the phone, alarm clock and music, but you might want to block email and texts.
In some cases, guests have limited access to your content. With the Gallery app, your collection of photos won't generally appear unless they are in the "Guest album." Guests can take photos, too, and have them appear there. On the other hand, if you enable access to the Photos app, your guest gets everything. Likewise, there are no restrictions with email or texts if you allow access to those apps.
I recommend logging in as a guest - with the alternative code - to verify what's available after you pick the apps to allow.
The bigger picture
These tips touch only the surface of what you can do to protect your privacy.
For instance, these apply only to data stored on the device. For files stored on Internet-based storage services such as iCloud and Dropbox, you'll want to make sure you have a strong password and turn on a second layer of protection, often known as two-step verification. I covered that in a previous column, which can be found here.
You'll also want to pay attention to what data you're sharing through apps.
With iOS, you can choose which apps can know your location and when, such as all the time or only when the app is actively running. Go to the "Location Services" settings under "Privacy." Unfortunately, it tends to be all or nothing with Android. You can turn off location services, but that affects all apps, including maps and others that might need your location.
With both iOS and Android, you can choose to limit ad targeting based on your interests and surfing history. For an explainer, read our column.