Google Photos is a newly launched application from Google that now backs up all your photos for free (as long as you're ok with their being 16MP and 1080p). But I also have Google Plus with Instant Upload enabled. Are these two completely different services, and is there any way to get my Google Plus Instant Upload photos (which is a lot) into Google Photos?
Google Photos is basically just a rebrand of Google+ Photos. Same data, different apps handling the backups.
Google Photos is Google+ Photos rebranded to be more generic (and not limited to the G+ network).
If you access Photos from your Google+ profile, it will redirect you to the new Google Photos service (photos.google.com), where all your photos from G+ will be. I noticed that after I first accessed Google Photos, it appears that it took several hours for all my photos from G+ to be available through the new service.
Google Photos is also more closely linked with Google Drive, with a specific option to import media from Google Drive.