An "article" is a piece of writing that's published in a newspaper or magazine. Sometimes writing that's posted on the Internet can also be called an "article", but sometimes it isn't.
Here are some types of writing that are not called "articles":
something that a person writes on their personal website is usually called a "blog"
a fiction story (a story about something that didn't really happen) is called a "short story".
when the main point of the writing is to review something like a movie or book, it's called a "review"
when a writer tries to explain a personal idea or experience that they have, it's sometimes better to call it an "essay"
Of course, there are many cases where it's OK to call something either a "blog" and an "article", or an "article" and an "essay".