A blog is also a website and a sort of an online journal or online diary where you can write and express your thoughts. You can write your personal experiences, stories and opinions, even your ramblings and rantings, that is if you like people to know about it. You can write news, reviews, commentaries and recommendations too.
Blog is a shortcut of “weblog”. A “weblog” is a coined word of “web” and “log”. The writer or author is called a “blogger”. Writing or authoring a “blog” is called “blogging”. The articles written are called “blog posts”, “posts”, “blog entries” or entries”.
What I wrote is according to what I have read.
According to Dictionary.com:
Main Entry: blog
Part of Speech: noun
Definition: an online diary; a personal chronological log of thoughts published on a Web page; also called Weblog, Web log
Example: Typically updated daily, blogs often reflect the personality of the author.
Etymology: shortened form of Weblog
Usage: blog, blogged, blogging v, blogger n
According to Wikipedia, the free encyclopedia:
A weblog, which is usually shortened to blog, is a website where regular entries are made (such as in a journal or diary) and presented in reverse chronological order. Blogs often offer commentary or news on a particular subject, such as food, politics, or local news; some function as more personal online diaries. A typical blog combines text, images, and links to other blogs, web pages, and other media related to its topic. Most blogs are primarily textual although many focus on photographs, videos or audio.
If you would like to have a blog, you need to read more in Wikipedia on how blogs are made, on how to create and maintain a blog. You can use a blog software like WordPress, Blogger, and Live Journal among others.