MAVLink messages are defined in XML files in the mavlink/message definitions folder. The messages that are common to all systems are defined in common.xml (only messages contained in this file are considered standard messages).
The common messages are provided as human-readable tables in: Common.
Vendor Specific Extensions (Dialects)
MAVLink protocol-specific and vendor-specific messages (dialects) are stored in separate XML files. These often include the common message definition, extending it with needed vendor or protocol specific messages.
While a dialect can include any other message definition, care should be taken when including a definition file that includes another file (only a single level of nesting is tested).
Vendor forks of MAVLink may contain messages that are not yet merged, and hence will not appear in this documentation.
The human-readable forms of the vendor XML files are linked below: