SOAP (Simple Object Access Protocol) and REST (Representational State Transfer) are two different approaches to web service communication used in the development of web applications. Each has its unique set of features, use cases, and design philosophies. Understanding their differences is crucial for developers to choose the most appropriate protocol for their application’s needs.

SOAP

1. Protocol: SOAP is a protocol, meaning it has a strict set of rules that must be followed to ensure interoperability between different systems. It uses XML for messaging and typically relies on HTTP for message negotiation and transmission, although it can work with other protocols as well.

2. Standards and Security: SOAP supports WS-* standards, which provide specifications for security, transactions, and messaging patterns. This makes SOAP a preferred choice for enterprise-level web services that require high levels of security and transactions, such as financial services and telecommunication services.

3. Envelope Structure: SOAP messages are wrapped in an "envelope" that can contain a header and a body. The header can include metadata and the body carries the actual message data, allowing for more detailed and structured information exchange.

4. Stateful Operations: SOAP can support stateful operations. While web services are generally stateless, SOAP services can be designed to maintain the state of information from one request to another.

REST

1. Architectural Style: Unlike SOAP, which is a protocol, REST is an architectural style for designing networked applications. It relies on a stateless, client-server, cacheable communications protocol — almost always HTTP. REST uses standard HTTP verbs (GET, POST, PUT, DELETE) to perform operations.

2. Format Agnostic: While REST can use XML, it is not limited to it. RESTful services often use JSON (JavaScript Object Notation) to pass data. JSON is lighter than XML, making it a better fit for web and mobile applications due to faster parsing and reduced bandwidth consumption.

3. Simplicity and Scalability: REST is simpler to use and easier to implement, making it widely popular for web APIs. It is inherently stateless, but applications can use cookies or tokens to manage sessions. This simplicity also contributes to REST's scalability and performance, particularly for internet-scale applications.

4. Resource-Oriented: REST is centered around the concept of resources, each identified by URLs. The representation of these resources can be manipulated via the standard HTTP methods, making it intuitive and straightforward to use.

Key Differences

• Communication: SOAP uses a single standardized XML format for all messages, while REST allows for multiple formats (like JSON, HTML, XML) and typically leverages the simpler JSON format.
• Flexibility and Performance: REST is more flexible and generally offers better performance and scalability, thanks to its lightweight messages and stateless nature. SOAP's comprehensive standards and protocol can make it more rigid but better suited for complex applications requiring high security and transactional reliability.
• Standards Compliance: SOAP adheres to stricter standards through the WS-* specifications, which can be advantageous for enterprise applications needing explicit protocols for security, transactions, and messaging patterns. REST, being an architectural style rather than a protocol, does not have built-in support for these but can implement them as needed.
• Use Cases: SOAP is often chosen for enterprise-level web services where security and transactional reliability are critical. REST is preferred for web APIs and services where simplicity, scalability, and speed are important, such as mobile applications, social networks, and cloud-based services.

Choosing between SOAP and REST depends on the specific requirements of the application, including factors like security needs, complexity, performance requirements, and the type of operations and resources being accessed.