The idea of Wikiportals, a page functioning as a portal for a particular subject area, was first introduced in the German Wikipedia in 2003. It was later incorporated in the French, Polish, Dutch, Japanese and Hebrew Wikipedias, but was not adopted in English until the concept was brought here from the Polish Wikipedia by Ausir.
Ausir opened the Biology portal on 5 February. This soon spurred the creation of portals for Art, Sports, and numerous other subjects over the past week.
While the larger French and German Wikipedias have more Wikiportals, the Polish ones are generally the most advanced in terms of visual design and layout. Since the Polish Wikipedia was also the most immediate source for the idea, the layout of Polish Portals has been copied for use in the English Portals.
Despite the jump in portal creations, their exact goal and their difference from WikiProjects remains unclear. In some other languages, the two exist side-by-side, often covering the same subject matter, and some of the new English Wikiportals have corresponding WikiProjects as well.
The Polish Wikipedia, however, doesn't seem to have WikiProjects (or at least the WikiProject page has no interlanguage link to Polish), which may be part of the reason why the portal idea was implemented there. But not all Wikipedias are clear about WikiProjects either.
The French page about WikiProjects indicates that they're supposed to be for coordinating issues across different Wikipedias. The German and English pages about WikiProjects say that they're for collaborating in a particular subject area within Wikipedia, with no allusion to other languages.
Portals are almost like mini-versions of the Main Page for a particular subject area, although their visual resemblance to the Main Page is stronger in other languages than English. WikiProjects seem to be more like working pages to decide on policies and conventions. However, a clear distinction between them isn't clearly mentioned in documentation on the matter in any language.
With respect to Wikiportals, there are also inconsistent practices across different languages, particularly involving their location. In the German and Polish Wikipedias, for example, the portals are part of the article namespace; French and now English, on the other hand, have them in the Wikipedia namespace.
On Saturday, Ausir posted a request to the wikipedia mailing list for a separate namespace to be created for Wikiportals. He said they don't really belong either in Wikipedia namespace or in the main article namespace. So far, there has been no response to the request.