Google+ was a social networking application on the web. It was discontinued in spring 2019.
Circles are one of the most interesting features of Google+.
Circles are the inverse of following. Following on Twitter means “I will listen to what you say” (and you can be a voyeur by not posting anything). On Friending on Facebook means “we will both be listening to each other” (and you can be a voyeur by not posting anything). On Google+ circling means “I might address you in my non-public posts” (and you can only be a very limited voyeur by reading my public posts). I can limit my posts to my work circle and the gamers in my RPG circle will never know. (The same can be achieved with a lot of clicking using Facebook lists.)
This “picking a target audience” is a Google+ benefit. But there is a drawback, too: reduced serendipity. If I add you to my RPG circle, you might never learn of my love for Emacs.
With blogs, you can sometimes subscribe to feeds for certain tags only. On Google+ we create collections. Collections indicate to readers: these are the topics I’m going to write about. You can subscribe these topics individually.