Wikipedia is the online encyclopedia that "anyone can edit." Unless you want to edit the entries on Albert Einstein, human rights in China or Christina Aguilera. Wikipedia's come-one, come-all invitation to write and edit articles, and the surprisingly successful results, have captured the public imagination. But it is not the experiment in freewheeling collective creativity it might seem to be, because maintaining so much openness inevitably involves some tradeoffs.

At its core, Wikipedia is not just a reference work but also an online community that has built itself a bureaucracy of sorts — one that, in response to well-publicized problems with some entries, has recently grown more elaborate. It has a clear power structure that gives volunteer administrators the authority to exercise editorial control, delete unsuitable articles and protect those that are vulnerable to vandalism.

Those measures can put some entries outside of the "anyone can edit" realm. The list changes rapidly, but as of yesterday, the entries for Einstein and Ms. Aguilera were among 82 that administrators had "protected" from all editing, mostly because of repeated vandalism or disputes over what should be said. Another 179 entries — including those for George W. Bush, Islam and Adolf Hitler — were "semi-protected," open to editing only by people who had been registered at the site for at least four days. (See a List of Protected Entries)

While these measures may appear to undermine the site's democratic principles, Jimmy Wales, Wikipedia's founder, notes that protection is usually temporary and affects a tiny fraction of the 1.2 million entries on the English-language site. "Protection is a tool for quality control, but it hardly defines Wikipedia," Mr. Wales said. "What does define Wikipedia is the volunteer community and the open participation."