"Blog" is an abbreviated version of "weblog," which is a term used to describe web sites that maintain an ongoing chronicle of information. A blog features diary-type commentary and links to articles on other Web sites, usually presented as a list of entries in reverse chronological order. Blogs range from the personal to the political, and can focus on one narrow subject or a whole range of subjects.

Many blogs focus on a particular topic, such as web design, home staging, sports, or mobile technology. Some are more eclectic, presenting links to all types of other sites. And others are more like personal journals, presenting the author's daily life and thoughts.

Read more: http://codex.wordpress.org/Introduction_to_Blogging

Getting familiar with Wordpress.com

Pages

Pages are static. They are a good way to publish information that doesn’t change much, like an About page. Pages appear in the Pages widget and in tabs/links across the top of some themes. Pages are not associated with a date/time like posts, nor can you tag or categorize a page.

At times, pages are confused with posts. See Post vs. Page for an explanation of the differences.

See more: http://en.support.wordpress.com/pages/

Posts

Posts are what make your blog a blog — they’re servings of content that are listed in reverse chronological order on your blog’s front page. Posts are always associated with a date, which is included in the URL. Sometimes, posts are confused with pages. See Post vs. Page for an explanation of the differences.

See more: http://en.support.wordpress.com/posts/

Categories

Categories provide a helpful way to group related posts together, and to quickly tell readers what a post is about. Categories also make it easier for people to find your content. Categories are similar to, but broader than, tags.

See more: http://en.support.wordpress.com/posts/categories/