Updated on August 16, 2017 on account of remarkable changes in the landscape of Google+. The post is up-to-date and current now. In this series on amazing reasons why you should be on Google+ and what you are missing if you are not, here goes the second one - the sequel to  Immersive Look-and-Feel. Enjoy the updated new post!
 Circles'Circles' is highly useful feature of Google Plus. Conceptually, it provides you a flexible way of sharing information - (same or) different posts with different set of people. A circle is nothing but a way of grouping people according to any criteria you wish. Using circles, you can control who you share your posts with.
Assuming you are logged into Google+ on your desktop, you can follow Menu on the left side for 'People' and once you are there, you will see the below screen. Mobile version is intuitive and once you are able to understand this feature, you should be able to navigate without difficulty.
|Google+: People and Circles|
You may want to create new circle/s adding people, for example, for collaborating on a project or say, for following on a particular interest like learning a new skill or hobby. You can create new circle by clicking on 'New Circle' link at the bottom right - indicated with red arrow. These people need not add you back in any of their circles. The public posts from the people in your circles appear in your home stream.
You add people to the 'Following' circle by default when you find people and click on the Follow button next to them:
|Google+: Follow from a person's profile|
|Google+: Move a person to a different circle|
Here, un-check the 'Following' circle and check the required circle where you want him/ her to add. When finished, click on 'Done' at the bottom in the pop-up and you have moved the person successfully!
You may face a common problem every beginner faces. You don't know where to start and whom to add. You can find suggestions in the 'Find People' tab for following and including people in any of your circles. Your suggestions come from your contacts whom you interact with on other Google services like GMail, etc. Besides you can search for people, hobbies, interests, etc. by entering the search term in the Search bar on top. Navigate around the search results and you will definitely find amazing reasons to Google+. You can continue with the 3rd part of the series: Google+ Communities. Enjoy. Have fun !
Original articleClassic Google+ at the time of writing my original post (way back in May, 2015), retired on January 24, 2017. Read this post from Google+ Product Manager announcing the same. Thought of keeping part of original article safe for benefit of history:
|How to Google+: Circles|
When you have logged into your Google-plus account (using your regular GMail account), you can view Circles by hovering on the top left button (encircled in red in the below image) and select People. You can then select Your circles and view and manipulate your circles. By default, you have 4 circles - Friends, Family, Acquaintances and Following - shown near the bottom in the image above (highlighted in red rectangle). You can create more circles as suitable to your needs by clicking on the grey circle with + sign to their left side. You can add (categorize) people into these circles and share different information with them. For example, you can share an article of general interest with everyone across all circles (Public sharing option). But you post your photos to the Family circle. Only the people in your Family circle will be able to view your post with photos. Next, it's not necessary for you to add me back if I have added you to any of my circles. Now this is a powerful feature. People who have added you to any of their circle can only see information you have shared as Public. This feature allows you to explore networking opportunities with almost everybody on Google+. They don't need to add you back if you have added somebody. Similarly you don't need to add anybody back if you are not willing to. This is superb! Then, there are situations when you don't expect somebody to add you back e.g. if you are following a celebrity, a technical magazine, your favorite brand, etc. You are interested in their posts in your stream and at the most, you may want to post your comments or share their posts. But you are not keen to broadcast your posts to them. There is a special circle - Following - for such people (or brands) whom you don't know personally but you are following. When you share your stuff with All your Circles, the Following circle is excluded. In case of a celebrity adding you back, you may shift her to a different circle - may be Friends - and she can see your posts meant for your Friends circle. Cool. Isn't it? Click here to learn about the 3rd reason: Google+ Communities...