Most simply, a blog is a way for you to put content up on your own website without needing anyone else to code or edit it for you.
From Wikipedia:
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. …
Many blogs provide commentary or news on a particular subject; others function as more personal online diaries. … The ability for readers to leave comments in an interactive format is an important part of many blogs. Most blogs are primarily textual…
Free blog services allow anyone to sign up for an account and keep a blog, updating it with whatever kind of content they choose.
Blogs can be used as websites or added on to a website to provide easily updated content.
