A blog is a nickname used by a blogger, which refers to both an online diary entry posted by someone on a web site and to the web site that they use to post such entries. It is a contraction of "web log".
Whilst the term "online diary" was used on web sites such as Open Diary, and the term "online journal" was widely used in Live Journal and its various spin offs, since WordPress, Blogger, Tumblr and Medium are now far more popular and all use the term "blog", that term is now the accepted term.
A blog entry itself can be anything from a report about news to something about what is going on in their day. There is essentially very little difference between a blog and a news report or a regular web page, however they differ because of their intention. The intention of a blog is that it is a temporary personal thing. Blog software is usually written in such a way that it posts a time stamp as well as the name of the person that posted it and is typically easy to use such that you just type about your day, as you would for a diary.