Many online journal articles are assigned a unique number called a Digital Object Identifier. The DOI can act as a permanent link to the article, so these articles can be tracked down easily.
Not all articles will have a DOI.
How do you find an article's DOI?
DOIs are commonly found in article records near other citation information (title, author, date of publication, source, etc.), or on the first page of the article itself.
e.g. Here's a record from a database that clearly shows this article's DOI:
And here's an example of a DOI found in the footer of an article's first page.
Can't find a DOI? If you don't see one then it's fair to assume your article doesn't have one. If you need to cite an article without a DOI, the library has examples for both MLA and APA style.