A wiki  is a means for collaboratively editing a Web site.
The idea is extremely simple: on every page on the Web site, there's an Edit link. Any Internet user can click the edit link, and edit the page in the browser. They can then save their version, and it's instantly shown on the Web site.
A wiki makes it possible for a loose-knit group of people to collaborate on creating a Web site. That's what we're trying to do here: create a free, complete, up-to-date and reliable world-wide travel guide. A wiki is the tool we're using to do that.
Because the barrier to editing pages is so low, people feel comfortable (we hope) making small changes or adding entirely new articles. The wiki doesn't require a long registration process. People can create user accounts if they want to have an identity in the community, but they don't have to if they don't want.
Nor is there any kind of editorial vetting process. Editing is distributed – anyone can do it. If anyone sees an error on a page, they can just fix it. That's it! Everyone's knowledge comes in in real-time – just when travellers need it.
Iteration and convergence 
Part of this low barrier to entry means that we can make articles through an iterative process – repeatedly editing an article to make it better and better.
For example, the first person to write about a subject might just create a stub, that is, just a sentence or two about the topic. The next person might add more, and the next person might add more to that. Someone may come in and combine a few sentences or paragraphs so they flow better. Someone else might come in and correct the spelling, and another person may come in and re-format the article to match our manual of style.
Gradually the article converges on a really informative, high-quality piece of writing. Sure, there may be some setbacks along the way, but bit by bit wiki articles get better and better.
Soft security 
Of course, anyone who's been on the Internet for a while is probably laughing into their sleeve. "Yeah, sure it's gonna get better and better," you might say. "Right. In Dreamland La-la-world, that may happen! In the real world, any article is going to have LIK MY BALLLZ!!! written across it by some goshdarn kid in less than two seconds."
This is a pretty reasonable concern. Of course, the Internet being what it is, this happens all the time. One of the great things about a wiki, however, is that anyone can change an article back to how it was before the abuse. Just as everyone is responsible for writing and editing the wiki, everyone is responsible for protecting it from malicious abuse.
Fixing an abusive edit – whether graffiti, an advertisement, or whatever – is just as easy as making the abusive edit in the first place. We keep a history of every version of every page, that anyone can look at. So, if a page just says, "I M THE GRATEST", another user can check the history, find the previous version, check that it's worthwhile, and then revert to that version. Easy as pie!
Sure, the abuser may just come back again and put back up their graffiti or whatever. But what seems to happen is that they don't, very often. They usually give up after one, or sometimes 2 or 3 tries.
Nobody's 100% sure why, but it seems like if there's a dedicated community that really cares about the wiki project – like Wikivoyagers care about Wikivoyage – then their efforts will outweigh any single abuser's determination. Abusers just don't care enough to spend a lot of time editing a page over and over just to see BIG BOB WUZ HERE on the Internet. It's not really worth the effort, and it's not that much of a challenge.
See also: How to handle unwanted edits
It happens fairly frequently that two or more contributors to a wiki site don't see eye to eye. Person A may think that Joe's Restaurant in Crane's Butte, Florida has really great chili; Person B might think it tastes terrible. Whose version goes into Wikivoyage?
The wiki's iterative process has its advantages in these situations. Person A may add a listing for Joe's to the Crane's Butte guide, and B may edit the article afterwards and take it out. A may put it back in. B may take it back out again, or B may change the recommendation for the chili to a critique.
Persons A and B would try to figure out what kind of wording meets our goals and is generally fair. We get to a point where the wording in the article is acceptable to everyone. Through editing and talking, we reach a consensus opinion – a version of the guide that gives the traveller the best information they need to make their own decision.
The downside of this method, though, is that daring, challenging, opinionated or controversial statements tend to get softened or elided out of articles. That's OK, though. Wikivoyage isn't an entertainment or fiction Web site. It's a travel guide, a reference book that gives practical, factual information to the traveller. It may not be as flamboyant as something written by a single person, but we think it will still have valuable, readable, accurate travel information.
Wiki markup 
The standard Web format for pages is HTML. HTML tends to be pretty wordy, and it can be hard to pick out the "real" parts of an article from all the HTML gobbledy-gook. This discourages people from editing pages – if they can't easily change the spelling of a word, they're probably not going to try.
For this reason, most wiki sites use a special editing format for their pages, called wiki markup.
The details of the format differ from site to site, but usually it's a simple set of rules for formatting a document. In the wiki markup that we use on Wikivoyage, for example, you can put a word in italics by putting two apostrophes ('') around it.
This can be kind of frustrating for people experienced with HTML, but it's really quite useful once you get the hang of it. And it does make the text of pages easier to manage when editing a page.
Another downside to wiki markup is that you can't do as much with it as with HTML. The standard wiki philosophy on this is to emphasize content over form – that is, what really matters is what's in the article, not how it looks. Just in case some really fancy formatting is needed for a page, however, the software we use for Wikivoyage does allow some HTML to be used. We try to avoid it unless really, really necessary.
Because everyone collaborates on every article, there's not a lot of concept of authorship in the wiki world. In particular, on Wikivoyage, we try to avoid using first person pronouns ("I", "we", "me") or qualifying statements with personal opinions or specific experience ("I think that...", "In my opinion", "When I was in Beijing last fall...").
Amazingly, clear, readable text comes out of the wiki iterative process more often than not. The weird part is that this is no one author's voice (usually), but the melding of a few or maybe dozens of contributors' voices.
This isn't to say that people can't gain reputation and respect for their individual achievements and contributions to Wikivoyage. We appreciate every bit of information, but the more people contribute, the better their reputation in our community. People who've done exceptional jobs writing, editing, and organizing a particular part of the site tend to gain status, and others will defer to their opinion. But they don't "own" any part of Wikivoyage, and they can't prevent anyone else from contributing. It's a win-win situation.
Another part of the wiki is that there's a distribution of ownership. We use a copyleft license to dot our i's and cross our t's with respect to copyright.
Each contributor, when they create an article, licenses that work to the public using a Creative Commons license. The next contributor who edits that article creates a so-called "Derivative Work", which they own the copyright for, and which is in turn licensed to the public – under the same license.
The license we use is very liberal – it allows anyone to copy, print, or distribute Wikivoyage articles in any way they want. The only rules are that they have to give attribution to the contributors who made it, and they have to share and share alike – they can't keep other people from copying, printing, or distributing the articles in the same way.
So, technically, the latest contributor to an article "owns" the article as it stands. In another way, everyone owns Wikivoyage articles – they can do practically anything an "owner" would do with their own stuff.
In practice, we all have a feeling of ownership for all of Wikivoyage. Wikivoyagers put work into all parts of the site, doing what they can to make it a better travel guide. We're all proud of the work as a whole, and of the part we played in making it.