The OpenStreetMap PBF ('ProtocolBuffer Binary Format') format is based on Google's Protocol Buffer ("ProtoBuff") format. As a binary format that does not include multiple redundant field definitions, this is much more compact than the older OpenStreetMap XML-based format. The ProtoBuff format is generally good for transferring large numbers of data records, making it ideal for OpenStreetMaps. It is also language-neutral, and platform-neutral - both essential characteristics for a file format that is intended to be used on virtually any piece of hardware or software.
Google's ProtoBuff Developer Site includes tutorials and sample libraries for reading and writing ProtoBuff files in C++, C#, Dart, Go, Java, and Python.