The Jagex cache is a bundle of files downloaded onto a player's computer the first time they load RuneScape. This cache is Jagex's way of storing game data. Data from the cache is unpacked when the client starts up, and is requested from the cache on login or when a player changes regions. It is possible to extract all of the raw game data from the cache, including but not limited to images, models, animations, textures, maps, and other miscellaneous game configuration.
After much discussion, the prevailing policy is that images derived from this cache may not be used on the Wiki. The closure of this thread, which notes that "the general consensus is that it may be breaking copyright law, and that it provides no distinct advantage for us to use these images", established this policy.