All three products i.e. Google+, Picasaweb and Picasa are Google products. All three products have different utilities.
Picasaweb is a website where registered users can upload & share photos with friends and family. The photos from computer are stored in "Web Albums" and a link to these albums is shared with others. Its like, the person who you share link with has the permission to see your web albums.
Picasa is a software program that when installed on your computer system allows you to edit, organize, and share your digital photos.
Google+ is the social networking site from Google. Whereas, "Google+ photos" is its photo section. It is exactly like Facebook Photos.
You can install PicBackMan's Google Drive uploader for Mac from the website and start backing up photos and videos.
Quick Tip to Keep your Photos Safe
Making backups of your photos & videos is a chore you probably skip, but it really doesn't have to be hard. Here's an effortless way to backup photos - PicBackMan. PicBackMan is the easiest and simplest way to keep your photos safely backed up in one or more online accounts. Simply download PicBackMan (it's free!), register your account, connect to your online store and tell PicBackMan where your photos are - PicBackMan does the rest, automatically. It bulk uploads all photos and keeps looking for new ones and uploads those too. You don't have to ever touch it.