In this application type, the most common use is probably as a personal notebook. So, what makes a wiki different from any other text-editing tool? We feel it's the totality of the features.
An Internet-aware database with free-form structure and integrated search function. Content is always "just a click away".
Accessability in the form of a transparent way to edit both content and structure (links) from any Web browser.
Portability, in that the database is not restricted to proprietary formats or particular operating systems.
A run-anywhere, access-anywhere resource. A properly configured wiki on a PC responds to requests just like any other server, from local or remote Web browsers. The database can also be relocated to any industry-strength Web server configured to run the appropriate wiki script.
We readily admit that writing large amounts of text in a Web browser form is not especially pleasant, but copying and pasting into the form from one's favorite word processor or editor is easy enough as long as one remembers not to style the text using the other program's features (which styling would be lost). The common ASCII format neatly circumvents program incompatibilities, but at the cost of not importing any styling codes.
Personally, the aspect of Wiki that I've come to appreciate most is the flexible and natural way content structure can grow and change with changing requirements and viewpoints. This is especially valuable when making entries that have at the time few obvious connections with existing content, and it's not at all clear where it will lead. A good example is making research notes.
When researching a subject for an article or a book, I often follow many disparate leads, online and off. I collect notes in a wiki as short comments, text extracts, and Internet hyperlinks. Often, there might be no immediate structure, but I need to write down the note and link in order to move on. In Wiki, I can start by noting it down almost anywhere. The note is never lost because it can always be found using the built-in search function.
Later, as more material is collected, a topic or page title may suggest itself, and relevant content can be moved, leaving an internal WikiLink on the original page. Relationships between different topics are easily noted by adding cross-links on respective pages. For that matter, changing or expanding relationship links is as easy as editing in new WikiWord links.
Sometimes, I need to note that some lead isn't yet researched. Creating an open (dangling) WikiLink as a placeholder then serves as a reminder of this branch or reference. Such links can be specially searched for to generate a list of all issues still unresolved. The link is never "broken," as it would be in a static Web page, but undefined--it conveniently leads directly to the edit form to enter new content.
Thus, Wiki functions as a writer's tool in many different ways--unobtrusive yet always at hand.