Okie.. This is for the starters. Most of the non-tech savvy web surfers are usually not aware of what exactly a blog is. They consider it same as a website. And to a great extend it is true.
Quoted from Wikipedia.org
“A blog (an abridgment of the term web log) is a website, usually maintained by an individual, with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse chronological order. Blog can also be used as a verb, meaning to maintain or add content to a blog.”
The major difference that a new user needs to understand is that a blog is a kind of web diary sorted by dates and a website is a collection of pages with interlinked content.
As a nature, blogs are periodically updated and that is what the blog readers expect. Blogs are loved by search engine because of their periodic updating capability and easy sorting / book marking functions.
Websites are favored by search engine because of their ability to generate static pages. But now, this difference no more holds. With blogging packages like WordPress, it is difficult to segregate between a blog and website. WordPress is free blogging software that let you create your blog in minutes and start posting. It combines the blog with websites by providing pages which can be modified to be used as Websites.
A large number of plug-in are available which offers variety of functions. These ranges from SEO, designing, added functionality among others. There are other blogging platforms like Movable type and Blogger.
Darren Rowse has a nice article on the introduction of blogging for new bloggers. It would be repetitive commentary if I went on stating the same again. Do check out the links given below, if you are new to blogging.