What is the difference between Google Photos, Picasa, Album Archive, Drive Photos?

The different "Photo" websites developed by Google are just different viewers using the same data in your Google account:
If you upload (or have uploaded) pictures in one of them, they will be available instantly in all of them, and if you delete a photo in one of them, they will be deleted everywhere.

Current products
  • Google Photos for desktop is a website where you can upload pictures from your computer into "Albums" and share them with friends and family. There is also basic editing functionality as well as some features that reduce the workload for you organizing your photos collection like automatic face tagging, auto-enhancing photos and videos, stitching panoramic pictures, etc.

Retired products
  • Google+ Photos was the "photos" section on the social network called Google+. The photos section was officially discontinued and replaced by Google Photos in 2015. More info: https://support.google.com/photos/answer/6142214?hl=en&ref_topic=6128818
  • Picasa3 is a software program that you install on your computer. You can edit and organize your digital photos and videos on Windows and MacOS computers. It offers easy-to-use features to upload pictures to Google Photos and Album Archive.
  • Picasa Web Albums (aka Picasaweb) was retired in August 2016. No enhancements or changes were made to Picasaweb since 2012 when Google's efforts were concentrated into creating Google Photos. Photos at Picasa Web Albums also appear at Google Photos. In Picasa Web Albums, you could sort photos by name and create public albums. These feature do not yet exist at Google Photos.
    • No longer supported by Google. Read more here.