The traditional act of caching involves a hidden treasure that one finds using the lat/long coordinates found on a website. However Opencaching is anything but traditional, they are turning the caching world upside down with their new Virtual Cache.
A virtual cache has more to do with a special location than a physical container. The primary object is to log a unique site without actually finding a physical log sheet. Often times virtual caches are located in public areas that prohibit items being left, such as national parks or historic sites. Here's an opportunity to also setup a multi-cache, by leaving a physical cache nearby (if possible) with directions to and information about the virtual cache.
To address the missing log book cache finders will often be required to provide a code phrase from the site, to verify that the location was visited. This could be a slogan from a sign or some sentimental text found on a monument, but whatever it may be, the one finding it will have surely gained a positive experience.