Google Photos is a cloud-based service for photo and video sharing and storage that can be accessed through dedicated mobile apps on iOS and Android or via a web browser. Not only does it allow you to back up and organize photos from your different devices in a single place, it also provides access to various useful features, like tools and filters to edit your photos and videos. Google Photos is also where your Google+ photos are stored.
By default, photos and videos stored on Google Photos are not available on Google Drive and vice-versa. This tutorial explains how you can fix this situation.
Access Your Google Photos Library from Google Drive
Sign in to Google Drive with your Google Account. Click on the cog icon located at the top-right corner of your screen and select Settings > General.
Scroll to Create a Google Photos folder and tick the Automatically put your Google Photos into a folder in My Drive checkbox:
N.B. Modifications made to photos stored in this folder won't be reflected on Google Photos.
Access Your Google Drive Photos and Videos from Google Photos
If you want to use Google Photos to view photos and videos stored on Google Drive, then you will simply need to enable a setting. Connect to Google Photos. Click on the Menu button located at the top-left corner (3 horizontal bars) > Settings:
Toggle the Show Google Drive photos & videos in your Photos library option to the on position: