Picasa is an online image organizing & photo sharing service & is owned by Google since 2004. Picasa web is now integrated into Google+ account. Picasa Web Albums acts as a middle man between Picasa desktop application and your Google+ account. If you are uploading photos & videos to Google+ account, then it will automatically appear in Picasa Web Albums. But, this is possible if you are using the same email address for both accounts & it is helpful to batch downloading photos. If you are looking for a step by step process to batch download photos from Picasa Web Albums, read below.
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.