WordPress is a platform intended to allow you to share your thoughts and ideas freely and easily with the world. However, there are options to publish to more limited audiences.
The first way to control access is to limit who can find your website, which is done by keeping search engines, like Google, from seeing (known as indexing) your site.
To do this, we'll start at the Dashboard.
Navigate to Settings > Reading.
Normally the box next to Search Engine Visibility is unchecked. If you decide to check the box, it will “Discourage search engines from indexing this site.” It will depend on the search engine to honor your “request”. Some search engines will simply ignore it. Obviously this is not a sure-fire way of keeping your blog private.
You also have options on individual posts to keep them private, so that only people who are logged in to your site can view a given post. You can also password protect posts with a password you supply. Choose the Private radio button to keep a post hidden behind the login, or choose the Password protected button and then type in the password you wish to use. Click on OK when you are finished. Then be sure you click the Update button to save your post with the new settings.
The plugin More Privacy Options adds more privacy options on your site.
Post Password Token allows readers to access protected posts without having to enter a password by creating secret token urls for the post. According to the plugin creators it allows you to “give password-protected posts a secret url that can be shared with [select readers]. The url allows your select audience to see the content without the confusion and hassle of an authentication form, while hiding the special content from search spiders and the wider-world. You can revoke secret urls at any time, so if a secret url gets to someone you don't want it to, you can simply invalidate it.”
Global Post Password is designed for you to set a single password for the site and then to just turn password protection on or off for specific pages or posts.
WP-Members is a free membership management framework for WordPress® that restricts content to registered users.
There are lots and lots of other plugins designed to manage passwords in different ways.