Circles are basically the way of grouping folks you follow much like lists in Facebook. Except folks don't have to "Friend" you to be able to add you to their circles. In a way, it's like people on Facebook being able to follow you if you have it enabled for your account. It sets it up so that they can see what you are posting publicly. If you never post anything publicly then they shouldn't be able to see anything.
If you have concerns about someone that has added you to their circles, your best bet is to block them for an added layer of protection. Information about blocking is at this link, but here are the directions.
- Open Google+.
- At the top of the page, use the search bar to find the person you want to block.
- Open their profile, then click the down arrow next to their name .
- Click Report/block [person's name].
- Check the box next to Block [person's name], then click Done in the lower right corner.