Your Facebook posts can be shared with your friends, friends and friends of friends, or anyone who views your profile.
You can select these sharing options on a case-by-case basis from the drop down menu at the top right of a post. On the mobile app, select "Edit Privacy" and select one of the options. From the desktop, just click the little friends or globe icon next to a post's time stamp.
If you want everyone expect a few people to see a post, you can choose to hide it from specific people by selecting "Friends except..." on the mobile app and "Custom" on the desktop.