There is a big difference between WordPress Posts and WordPress Pages, the first ones are standard published on a timeline; the latter are static information pages.
If you want to blog and create an online dairy, your primary elements on your website should be Posts. Posts can be grouped together by Categories and Tags. Pages, on the other hand, cannot be included in those category and tag listings.
If you ever wanted pages to be included into those listings, you need a plugin to do that.
In general, all information that is not going to change much is a good candidate for pages.
Posts are daily entries or news entries, like mentioned before, in chronological order. So the newest entries are shown first. A post is always connected to at least one category. If you don't choose a category for a post, it will automatically be assigned to the standard category of your WordPress installation.
This is also the reason why you should change the name of the standard Uncategorized category.
Posts can be tagged with Keywords / Terms that are mentioned in the post's text. The tags also connect posts together to Tag listings.
Best practice for your WordPress site would be assigning posts to only one category. Your tags should be just one word if possible and should not be the same as the categories you have set up.
Pages in combination with Menus are your best option of you want to create a completely static website. You can create main pages and child pages in different layouts (if supported by your theme). Pages can be sorted into specific order and shown in a widget for easy navigation.
With the new menu options, you can create a complete site navigation solely based on pages.
You could use pages for an About section, but also about the services you offer, the history of your company or site subject and more. Your limit is your imagination!