1. Runtime Environment: Node.js is a runtime environment based on V8 JavaScript engine, providing a non-blocking, event-driven architecture that makes it suitable for building fast, scalable network applications. In contrast, Orleans is a distributed systems framework that provides a programming model for building distributed applications with a focus on scalability and fault tolerance.
2. Concurrency Model: Node.js uses a single-threaded event loop to handle asynchronous I/O operations and callbacks efficiently, making it ideal for I/O-bound applications. On the other hand, Orleans uses a virtual actor model that allows developers to write code without worrying about the complexities of managing concurrency, making it suitable for CPU-bound applications.
3. Programming Language Support: Node.js is primarily used for writing server-side applications in JavaScript, while Orleans supports multiple languages such as C#, F#, and Python. This difference in language support can influence the choice of framework based on the existing skill set of the development team.
4. Scalability Approach: Node.js achieves scalability through clustering and load balancing, where multiple instances of the application run simultaneously to handle incoming requests. Orleans, on the other hand, uses a distributed actor model that automatically distributes and load balances computation across a cluster of machines.
5. Fault Tolerance Mechanisms: Node.js relies on external modules or custom code to implement fault tolerance mechanisms such as process monitoring and automatic restarts in case of failures. In contrast, Orleans provides built-in features for handling failures, including transparent actor state persistence and automatic actor activation and deactivation to recover from failures.
6. Use Cases: Node.js is commonly used for building web servers, API servers, and real-time applications such as chat applications. Orleans is more suitable for building distributed systems, IoT applications, and multiplayer games that require scalability and fault tolerance at the core of their design.

In Summary, Node.js is a runtime environment for building fast, scalable network applications using JavaScript, while Orleans is a distributed systems framework focused on scalability and fault tolerance through a virtual actor model.