A Planet file is a dump of the entire OpenStreetMap database to an XML file, containing the latest version of each feature in the world at the time it was produced. If you need OpenStreetMap data in significant quantities, downloading a Planet file is the best way of getting it, and for very large quantities of data, it is the only way of doing so.
We'll go through how you can obtain a planet file or other files based on it here. We'll cover the tools you can use to process a planet file in the next chapter.
All Planet files are initially stored at http://planet.openstreetmap.org/. At present, one full planet file is produced every week, usually on Thursday morning GMT. Between full Planet files, a series of files containing the edits made to the database are produced, called diffs, short for differences, after the files used to change the source code of software. OpenStreetMap diffs use a customized file format, and you have to use tools specifically written...