A weblog, which is usually shortened to blog, is a type of website where updates, or posts, are usually made and displayed with the most recent post being at the top of the page.
While many blogs are personal in nature and act as a public online journal they can really be about anything. Often you will find blogs dedicated to a specific topic such as food, politics, or local news. A typical blog combines text, images, and links to other blogs and web pages related to its topic. Some blogs are dedicated to a specific media. Video blogs, audio blogs, and photoblogs have all become popular. There are even moblogs (short for mobile blogs) which are blogs written by a cell phone or PDA.
Interestingly, the term "weblog" was coined by Jorn Barger way back in 1997. This was later shortened to "blog" and is accepted as both a noun and verb. So you can say, "I have a blog." or "I blog about cars."