So, now you've logged into your site, now what? Each site may be a little different, depending on how it's set up and what version you are using, but below is a general outline of how WordPress works.
The first screen you see is the dashboard -- the home page for your site. In the middle of the screen are boxes basically outlining what's going on your site.
Down the left hand side of the screen is the menu -- that's the important thing. This is where you navigate the site. Although there's a lot of options, the two areas you'll use the most are Pages and Posts. Pages and Posts are similar to each other, both have content, work the same way. However, there is an important difference between them.
Pages = Site
Pages are pages on your website. This is the static content that doesn't really change. The home page, contact page, services etc. These are the pages that usually go on the menu.
Posts = Blog
Posts are really your blog. This gets updated, changed, altered etc. They don't go on the menu as a rule, and you can go from one blog entry to another.
And the Rest?
Honestly, they're not all that important when you are setting up your site. Once you get the hang of Pages, Posts and uploading images, you're good to go!
For More Information:
Admnistration Area: http://codex.wordpress.org/Administration_Screens