OBD-II standards fall into two categories: generic and manufacture enhanced. The generic features are common to all manufactures. SAE (Society of Automotive Engineers) J1979 defines the message protocols used for vehicle communication. The available vehicle parameters are addressed by "parameter identification numbers" or PIDs. Wikipedia On-Board Diagnostics provides an excellent overview.
Manufacturer enhanced OBD-II PIDs are features unique to a manufacturer and are not standardized across all vehicle makes/models.
The generic OBD-II standards are readily available for purchase at a low cost. However, the manufacturer enhanced protocol information is not easily obtained. The most common question is how do I get these standards and how much do they cost? The answer is Equipment and Tool Institute is an organization that interfaces with all the manufacturers and combines all of the information for tool manufacturers to purchase. Every year new data is published for the subscribing members. The cost was $7,500/year to obtain the manufacturer specific OBD-II protocol information.
The data contained is raw data from different vehicle manufactures (Ford, Mazda, Toyota, etc…). The data is in different formats and is sometimes difficult to decipher into an actual software implementation. However, with the enhanced protocol specifications, it’s possible to design scan tool (or other devices) to communicate with the vehicle to command/query manufacturer specific features that are unavailable on using the generic OBD-II feature set.