In many ways, posts and pages are similar. However, there are a few key differences that you will need to bear in mind when you are creating content.
Most of the content you create on your WordPress site will be in the form of posts. They are a defining factor of the blogging format – if you are using a traditional blog layout, they will be listed in reverse chronological order on your home page. Additionally, visitors to your site can leave comments on your posts.
You are able to assign posts to categories, and you can also “tag” them with relevant keywords. They will display in category lists, archives, and RSS feeds.
You can manipulate the display of posts in many ways – for instance, you can install widgets (find out more) that displays the most popular or recent posts (I have not included a link to a recent posts widget because it is already available in a standard WordPress install).
“One-off” things such as About pages and Privacy Policies are usually published as pages. They will typically be listed in your navigation bar, although you can also display them in a widget.
You will probably only have a few pages. You will have lots of posts.
Creative Commons photo courtesy of e³°°°