Only the semantics and libraries implementing them are there. The wire format is not specified, which is a pretty annoying problem to deal with.
An analogy is SLF4J for Java logging. All libraries, etc use the same interface and the final user determines the backend: java.util, Logback. This makes sense if you have many authors of libraries with a cross-cutting concern.
This really makes OpenTracing half a dozen different standards, one per language, with common semantics.
Should it be about a wire protocol instead? Discussion at https://github.com/opentracing/specification/issues/34