According to "RESTful Web Services" by Leonard Richardson and Sam Ruby.

• Addressability

Every resource has its own unique URI. Suggestion: Every representation of a resource has its own URI. Headers shouldn't be the only tool a client has to specify which resource or representation is selected.

• Statelessness

The server never stores any application state. Each client request is considered in isolation.

• Safety

A client that makes a GET or HEAD request is not requesting any changes to server state.

• Idempotence

Making more than one PUT or DELETE request to a given URL should have the same effect as making only one.

• New Resources: PUT vs POST

A client can only use PUT to create resources when it can calculate the final URI. If the URI is not known, the client can POST to a "factory" resource.

• Connectedness

Instead of assuming that rules how to construct URIs are known, use links. A service ought to be self-describing, and not relying on auxiliary English text that tells programmers how to write clients.

• Relationships Between Resources

Instead of modifying the state of two resources, treat the relationship as a third kind of resource. The same goes for transactions. "When in doubt, make it a resource."

• URI Design

URIs should be meaningful and well-structured. Use query variables only to suggest arguments being plugged into an algorithm. It is almost never appropriate to put the names of operations in URIs.

• Outgoing Representations

Should be human-readable, but computer-oriented.

• Incoming Representations

Usually key-value pairs, form-encoding is the most popular representation. If a resource state is too complex for key-value pairs, the input format should be the same as the output format.

• Service Versioning

Simplest way: Incorporate versioning information into the resources' URIs.

• Permanent URIs Versus Readable URIs

Whether URIs as UIs (meaningful, may break when resource state changes) or opaque URIs (e.g. using only database id, permanent) are more appropriate depends on the clients.