Facebook was originally based upon a simple model of symmetric friend links, this means that for if Tom is Jane's friend then Jane must also be Tom's friend. It is for this reason that when you add a new friend on Facebook the other person must confirm that they also wish to add you as a friend. This simple model of friendship works well for the friendships that we establish in school (and hence it is a good match for Facebook which originally was most used within a school setting). However, real world relationships are much more complex and hence mode advanced models of social relationships are sometimes needed.
Not all friendships are the same. For example, most people would have different criterion for what they would be comfortable sharing with their work colleagues and what they would like to share with their close friends and family members. For this reason, Facebook introduced a feature called 'lists' where you can divide your friends into different catergories. When you share something on Facebook you can choose whether you want to share it with all of your friends or else restrict it to only the members of a particular list. Since this feature was not originally in Facebook when it launched many users are not even aware of its existence and hence they tend to share everything with all of their friends (and this is the default anyway).
The other weakness in the Facebook friendship model is the fact that it is not necessary true that the people whose updates you would like to read are the same as the people whom you are happy to allow read what you post to the site (Facebook uses your friend list for these two different purposes). Systems like Twitter recognise this difference by implementing asymmetric links that they call 'followers'. By default all updates sent to Twitter can be read by anyone. If I add someone to my 'follow' list it simply means that I would like their updates to appear in my default stream and does not imply that there is any relationship between us. For example, I follow the Dalai Lama on Twitter because I am interested in reading the profound thoughts that His Holiness posts to twitter, but I have no reason to think that His Holiness is interested in reading the boring stuff that I post. For this reason the Dalai Lama has over 2.5 million followers on Twitter, although he doesn't follow anyone.
Google circles are asymmetric links (like Twitter) because you can add anyone you want to your circles and there is no need for them to approve this and/or add you to one of their circles. However, Google circles are not like Twitter follow lists because circles are used to control who can read your updates as well as whose updates are presented to you in your default stream. Everytime you post something to Google+ you are asked to specify which of your circle's members should be allowed to read the update (there is a special group called 'public' that you can share with if you don't care who reads this update).
By default Google+ shows you all of the updates you are allowed to see from any of the people in any of your groups, but you can also click on a circle name to only show updates from people who are in that circle. Therefore in order for you to see a particular update it will be necessary that you have added the person who posted the update to one of your circles and it will also be necessary for you to be a member of one of the circles that the person posting the update chose to share it with. Hence the logo shows intersecting circles.