Before Google+ and Google Photos, Picasa was Google's photo sharing and management product. You may not be aware, but it has continued to exist all this time—well, languish may actually be a better term. There have been no improvements to Picasa lately, and now Google is closing it down.
The good news is that any photos you've added to Picasa over the years are already available in Google Photos. The two products can access the same image database. If you don't want to use Google Photos, you will still be able to see and download your existing Picasa web albums along with the associated tags, comments, and captions. You just won't be able to add any new albums or organize existing ones. Web albums will continue to work normally until May 1st, 2016, so you've got time.
Long before that, Google will stop supporting the desktop Picasa app on March 1st, 2016. You can download before then and it will continue to work going forward, but Google won't be developing it further. It'll probably start doing weird things when web albums become read-only in May. Google has been working on newer photo products for a few years, so I don't think Picasa users will be missing out on anything by switching over. It's probably time.
- Google Photos Blog