A journal is a published collection of articles regarding a particular subject, usually published on fixed dates much like a magazine. Many journals are bi-monthly while some only come out quarterly.
Some organisations now publish electronic editions of their journals, either as an alternative to the paper version or instead of it; however despite opting for a web-only format the early adopters are as a rule still releasing sequential editions rather than uploading individual articles as soon as they become available.
Within a single journal issue there will be a small number of articles, each focussing on a particular subject where the author (or, as is more common in scientific journals, authors) is revealing their research or findings on the topic.
The Academic Publishing Wiki is a wikicity exploring writing professional-level journal articles on a variety of subjects entirely in a free-text wiki environment.