The name "Picasa" is the combination of Spanish painter "Pablo Picasso". In July 2004, Google bought Picasa and began offering Picasa for free download.
Picasa has different versions there is the web version of Picasa which has the web.Picasa, PC version that is application software.
This photo service was start 12 years ago by Google for a different form and different versions. However, Picasa and it's related version Picasa Web Albums service will be discontinued in May. Google is now focusing on Google Photos, it is the smart way to share your photos which were released with Google+. Read more about Google Shutting Down Picasa
Google PhotosGoogle Photos is the app which comes in different versions where you can share your videos and photos and all of the memories.
It was announced in May 2015 and spun out from Google+. Where Google+ is the Google's social network.
Now Google is migrating all the photos from the web.Picasa albums to Google Photos because they are shutting down the Picasa.
March 15 2016, Google will no longer support the Picasa desktop app and Google will create new place to access your all the web.Picasa albums.