From a long time spent working with print magazines:
Article: intended to be used "as is"
Press release: intended to be used as input to editorial content.

Lazy editors use press releases "as is" and PR guys often try to write the editorial in the press release so there is a lot of blurring.

An article is written to be informative about a specific topic and a press release is more of an announcement for a person or business.

Unfortunately SEOs have blurred the lines... I believe SEOs have gotten tired of waiting of sites like EzineArticles to approve their articles so that they can then be republished on other sites, so they started spamming press release sites with actual articles to get faster results.

But as BrickMarketing said, Press Releases were originally meant for newsworthy announcements about a company.