A blog (a portmanteau of web log) is a website where 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.

Many blogs provide commentary or news on a particular subject; others 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. The ability for readers to leave comments in an interactive format is an important part of many blogs. Most blogs are primarily textual, although some focus on art (artlog), photographs (photoblog), sketchblog, videos (vlog), music (MP3 blog), audio (podcasting) are part of a wider network of social media. Micro-blogging is another type of blogging which consists of blogs with very short posts. As of December 2007, blog search engine Technorati was tracking more than 112 million blogs.

The emergence of blogging has brought a range of legal liabilities and other often unforeseen consequences. One area of concern is the issue of bloggers releasing proprietary or confidential information. Another area of concern is blogging and defamation. A third area of concern is employees who write about aspects of their place of employment or their personal lives, and then face loss of employment or other adverse consequences.

Few personal blogs rise to fame and the mainstream. That does not mean that authors of personal blogs do not take their blogging seriously. Personal bloggers take pride in their blog posts, even if their blog is never read by anyone but them. It is here that blogging becomes more than a way to communicate and becomes a way to reflect on life. Blogging can have a sentimental quality. Bloggers post about their day or their opinions on different matters. Personal blogs may not be important to readers, but for the people who write them, they are works of art, for example:- Search Engine Concepts

One unfortunate consequence of blogging is the possibility of attacks or threats against the blogger, sometimes without apparent reason. Kathy Sierra, author of the innocuous blog Creating Passionate Users, was the target of such vicious threats and misogynistic insults that she canceled her keynote speech at a technology conference in San Diego, fearing for her safety. [50] While a blogger's anonymity is often tenuous, internet trolls who would attack a blogger with threats or insults can be emboldened by anonymity. Sierra and supporters initiated an online discussion aimed at countering abusive online behavior,[51] and developed a blogger's code of conduct.

Several blog search engines are used to search blog contents (also known as the blogosphere), such as Bloglines, BlogScope, and Technorati. Technorati, which is among the most popular blog search engines, provides current information on both popular searches and tags used to categorize blog postings. Research community is working on going beyond simple keyword search, by inventing news ways to navigate through huge amounts of information present in the blogosphere, as demonstrated by projects like BlogScope.

Several online communities exist that connect people to blogs and bloggers to other bloggers, including BlogCatalog and MyBlogLog. A collection of local blogs is sometimes referred to as a Bloghood.