A spun-off product formerly integrated with Google+, Google Photos is a photo backup and viewer service by Google that comes with build in photo editor interface.
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Pro Free unlimited storage
Google Photos comes with free storage for images up to 16Mp and videos up to 1080p which means that most users need no longer worry about storage space.
Pro Can automatically back up phone's photos
Any images taken with your phone can automatically be backed up to Google Photos. And the first time you run the mobile app it will also help you back up any existing images and videos you might have on your phone.
Pro Can search photos
Text in photos, the content of the photo, and so on can be automatically searched. So one doesn't need to categorize to later find photos. Manual categorization is still possible.
Pro Non-destructive built-in photo editor
Any photos that you edited can be undone to original copy.
Pro Can manually classify photos
Can create albums from photos or even manage the whole folders tree manually via Drive.
Pro Send images and videos to Gmail and YouTube
If you linked your Google drive with google photos, then you can easily attach any of your Google Photos in an email message.
Con Might be removed anytime
Google has a tendency to quickly shut down or abandon services that don't perform well. Google Photos might run the risk of being discontinued if the user base of the app doesn't meet certain criteria.
Con The website has performance issues
The site tends to lag and file uploads can sometimes fail.
Con Not a lot of free storage space for high-res content
While there's a free unlimited storage plan, it's limited to 16 megapixel images and 1080p videos. Uploading full-resolution content, if you opt out of free unlimited storage plan, is limited to 15GB that comes with a Google account which is shared across all Google services. Increasing that storage space costs about $10/mo per 1TB of storage.
Con Doesn't backup in the background
For iOS devices
Con The web version lacks certain features available on mobile apps
Con Files may be compressed if you're using the unlimited storage option
All images exceeding 16MP and videos exceeding 1080p will automatically be compressed to that size or lower. Meaning high-res images and videos won't work with the unlimited storage option.