OpenTelemetry Java is a language-specific implementation of OpenTelemetry that provides an API and SDK for instrumenting code, as well as a Java agent that can automatically capture telemetry data. The SDK defines the implementation of the API, which can be used to configure how instrumentation data is processed and exported. Manual instrumentation allows developers to add custom instrumentation using the APIs and SDK, while auto-instrumentation uses the Java agent to modify bytecode and capture data without manual configuration. The OpenTelemetry project provides a range of examples and documentation for instrumenting Java applications with tracing, metrics, and other features.