Just like print journals, electronic journals are produced by a variety of publishers. These publishers can either be professional societies or commercial companies. Journals can also be provided collectively through a commercial aggregator company such as ProQuest, which negotiates with individual publishers to provide multiple titles. Each provider of a title creates the method by which you interact/interface with them and determines the format of the actual text. The current standard text formats include ASCII (plain text without graphics), HTML (looks different than the print, prints more pages than the print equivalent, and has hyperlinks) or PDF (looks just like a snapshot of the print, and requires Acrobat Reader).
In addition to the interface and text format, the number of years covered by an ejournal varies, as does the currency of the information provided. Aggregators, like ProQuest, are often several weeks to a month behind since they acquire the data from the publishers and reformat it into HTML. Conversely, some publishers are providing the electronic copy prior to the equivalent print issue.