You can add content to your site, by creating page or post. The procedure for both is almost identical. The difference is only the display on the site. The posts will be displayed under the notes, following the chronology. You can associate them with Categories and Tags whereas Pages don’t. The Pages will contain information where the timeline aren’t important, and where the content of the page is more fixed information eg.: reports, forms…
Imagine that your site is two books, one of them is like a diary this where your posts will be published, and here the Categories are like the Table of the Content and the Tags are like the terms in the index. Until the pages will be like a novel, where every page is a chapter, and you can group them under menu points as you do that with the books on the shelf.