Picasa is described as 'Software download from Google, that helps you organize, edit, and share your photos. The local application usage and the online storage up to 1 GB is free, and it is easy to use. The online version is called Picasa Web Albums' and is a leading Photo Manager in the photos & graphics category. There are more than 50 alternatives to Picasa for a variety of platforms, including Windows, Mac, Android, Web-based and Linux apps. The best Picasa alternative is digiKam, which is both free and Open Source. Other great apps like Picasa are FastStone Image Viewer, Ente Photos, Fossify Gallery and Adobe Lightroom.
Google Photos DOES NOT contain all the cataloguing, browsing and editing functions of Picasa on a local machine, i.e. no virtual albums, tagging, facial recognicial or geotagging. It is a replacement for the Picasa Website, not the main Picasa desktop application.