1. Programming Model: Dapr is a runtime for building distributed applications using a microservices architecture, while Orleans is a virtual actor model framework. Dapr allows developers to write applications using any programming language and provides a set of building blocks for common features like service-to-service invocation, pub/sub messaging, and state management. Orleans, on the other hand, enables developers to build high-scale, fault-tolerant systems by allowing them to write business logic using virtual actors, which is a higher-level programming model compared to traditional actor models.
2. Deployment Flexibility: Dapr provides a flexible deployment model where applications can be deployed in containers, such as Kubernetes, or as standalone processes. It can also seamlessly integrate with existing applications without the need for a complete rewrite. Orleans, on the other hand, is designed to run on clusters of machines and relies on its own hosting environment for deployment. It provides features like automatic load balancing and activation, making it suitable for highly scalable and distributed systems.
3. State Management: Dapr provides a state management building block that allows developers to store and retrieve state in a consistent and reliable manner. It abstracts away the underlying storage provider and supports features like strong consistency and optimistic concurrency. Orleans, on the other hand, provides a built-in distributed actor virtual memory (DAVM) system that allows actors to store and retrieve state locally. It also provides support for distributed in-memory caching for performance optimization.
4. Service-to-Service Invocation: Dapr provides a service-to-service invocation building block that allows applications to invoke other services using a simple API. It takes care of service discovery, retries, and circuit breaking, making it easier to build resilient and scalable applications. Orleans, on the other hand, uses a message passing mechanism for communication between actors, where messages are sent asynchronously and processed by the receiving actor.
5. Pub/Sub Messaging: Dapr provides a publish/subscribe building block that allows applications to publish events and subscribe to them. It supports multiple pub/sub systems, such as Apache Kafka, RabbitMQ, and Azure Service Bus. Orleans, on the other hand, does not provide a built-in pub/sub messaging mechanism. However, it can integrate with external pub/sub systems by using the observer pattern to notify actors of events.
6. Scaling and Sharding: Dapr provides built-in scaling and sharding capabilities, allowing applications to scale horizontally by adding more instances of a service, and to distribute data across multiple instances using consistent hashing. Orleans, on the other hand, provides automatic activation and deactivation of actors based on demand and allows developers to define custom activation strategies for fine-grained control over scaling and sharding.

In Summary, Dapr is a runtime for building distributed applications using a microservices architecture with a focus on providing a set of building blocks for common features, while Orleans is a virtual actor model framework that allows developers to write high-scale, fault-tolerant systems using a higher-level programming model based on virtual actors. Dapr provides more flexibility in terms of programming model and deployment options, whereas Orleans provides specialized features for state management, messaging, and scaling/sharding in the context of virtual actors.