A blog is a website that's like a diary.
To illustrate the differences between a website and a blog, I've created these two example sites on my FatCow hosting. They both use WordPress's standard TwentyEleven graphical theme.
|Is a structured presentation of web pages. Web pages are typically carefully-written pieces of information.||Is a collection of "posts". Posts are typically casual pieces of writing and news.|
|Each page is linked to from the menu bar.||Posts have no fixed location. New posts are added at the top of the main page, and automatically arranged into categories by topic and/or date. Older posts are moved to an archive.|
|Visitors are typically not permitted to post comments on pages.||Visitors are typically allowed to write comments on posts. The post's author typically joins in this discussion. (Allowing comments is optional.)|
|Appropriate for businesses, information sites, or any site where information is not time-sensitive, and doesn't go out of date.
Examples: A furniture store, a gardening information site, a parenting resource site.
|Appropriate for any group or activity that's casual or where information is time-sensitive news.
Examples: A blog about current events, a blog about the latest technology, a blog about recovery from an illness.
There's no exact line between what's a blog and what's a website. A website can have a blog page somewhere, and most blogs also have at least a few normal web pages, such as an "About Us" or "Contact Us" page. If the main page of the site is a page of the latest posts, then I'd call the site a blog.
By default a WordPress site is set up like a blog, with a blog page as the front page, and one regular web page as well. You can easily change this, however.