This article is for non-technical users who contribute content to sites for which WordPress has already been chosen as the CMS.
The essence of WordPress is the blog, though you can create WordPress sites that don't have them. Still, you need to understand the blog features and how to use them for blogs and non-blogs.
WordPress Pages (with a capital "P") are old-fashioned webpages. They began as parts of a trifold brochure. Then in the 1990s or so they were put on the web. They get edited every once in a while, but essentially they sit there. Pages hold the "About Us" information of a site: location, hours, staff, mission, etc. In WordPress, Pages can have parent Pages to create a hierarchical structure. The navigation menu of most sites is made of a list of Pages.
WordPress Posts are timely entries on the site: announcements, upcoming events, pictures of last week's event, reflections on current news. They usually show up in a list of most recent posts and an archive of posts by month and year. Like Facebook statuses, the older ones fall to the bottom and you see the newest information.
Posts can also be used to organize similar chunks of information. For example, an FAQ page could be a list of Posts marked with the category "FAQ." Even though the FAQs are not timely (they will stay around a while and aren't sorted by date), making each one a Post allows content contributors to easily edit them and designers to easily control how they are displayed.
There is a special type of Post known as a Sticky Post. Sticky Posts appear in the same list of Posts as others, but they stick to the top of the list until you un-stick them. To make a Post Sticky, edit the Post and go to the Publish panel. Where it says "Visibility: Public" click "Edit" to see the option.
WordPress is designed to allow your website visitors to Comment on Posts and Pages. At first, most people think they won't want to allow that. But stop a moment and consider. This is the heart of Web 2.0 — the ability for users to interact with your site. The more freedom you allow, the more vibrant your site can become. There are settings to allow you to turn off commenting or to moderate each comment before it shows on the site. Another solution is to allow other readers to mark a Comment as inappropriate.
WordPress has a special section for making Links. In a blog, this is traditionally known as a blogroll: blogs with similar content that you recommend. Use Links to link to related organizations and sites of general interest to your readers.
There is also a section for uploading Media. Media covers photos, other graphics, videos, and any PDF files or Microsoft Office documents you'll use on your site. You can upload and attach any of these to a specific Page or Post. When you know you will use a file repeatedly, upload it to the Media section and it will be available when you are writing Pages and Posts. The Media section is also useful for creating galleries of photos.
To add Media to a Page or Post, edit the Page or Post. At the top of the editing screen you'll see "Upload/Insert" with some cute icons. When you hover over them they tell you what they're for. Click one to add the type of Media you need. A pop-up will allow you to upload a file or to choose one that's already in the Media Library. Make sure to add a meaningful title for the search engines.
Categories and Tags
As you write Posts, you will probably want to provide some structure and search capabilities beyond just viewing Posts in the order they were written. That's where Categories and Tags come in.
Categories are boxes you set up ahead of time to group your Posts. You can have hierarchies of subcategories and you can assign a single post to more than one Category.
Tags are keywords from your Post. You tag the most important words and concepts in the Post so that the search engine finds Posts that are really about whizbangs instead of just Posts that mention whizbangs in passing. You usually don't set up Tags ahead of time. You can assign as many Tags as you want to each Post. Tags are less structured, which is why you often see them displayed in a Tag Cloud: a mishmash of words (Tags) with the most common ones shown in a larger font.
At the top right of your screen, you'll see Screen Options. There are many options to show in the admin screens and you probably don't want to see them all. Click the dropdown to check or uncheck the options. There is a set of options for the Dashboard and another set for Posts and one for Pages. There's a chance that your adminstrator has set this up nicely for you, but it's probably just set on default.
Now go forth and create some wonderful content!