A blog (a truncation of "weblog") is a discussion or informational website published on the World Wide Web consisting of discrete, often informal diary-style text entries (posts). Posts are typically displayed in reverse chronological order, so that the most recent post appears first, at the top of the web page. Until 2009, blogs were usually the work of a single individual, occasionally of a small group, and often covered a single subject or topic. In the 2010s, "multi-author blogs" (MABs) emerged, featuring the writing of multiple authors and sometimes professionally edited. MABs from newspapers, other media outlets, universities, think tanks, advocacy groups, and similar institutions account for an increasing quantity of blog traffic. The rise of Twitter and other "microblogging" systems helps integrate MABs and single-author blogs into the news media. Blog can also be used as a verb, meaning to maintain or add content to a blog.
—— From the Wikipedia.
So, in short, blog is a discussion / information displaying site in public ( sometimes it can be private ).
And there are many blogging systems, so bloggers do not have not create a system themselves and not need to learn complex coding languages.
Instagram, Facebook, Weibo, etc. they are big companies' blogging systems, besides these, bloggers could also choose using weblog like WordPress, Gitbook, Typecho, etc. these blogging systems is less frequently used by the public or people that not familiar with computer, networking and coding, and have less limits than those big companies' blogging systems, such as you can have your own css styles, add codes, etc.