I have googled around quite a bit still am confused as to what each of the above exactly mean.
Here is my understanding of it:
I downloaded each jar and tried to decompile and see what is inside it, but I am only able to find interfaces in all of them and not the implementation.
I am facing these questions in the context of duplicate warnings generated by maven shade plugin, and need proper understanding of the above to figure out which ones to exclude and why.
I'll first get to the question
"JSR311 it is a specification request. Which means it is supposed to be a document. Why then is it a jar?"
Except the last (jersey-core), all those jars are "specification" jars. The JAX-RS (as well as many other Java) specifications define contracts (or interfaces) that implementators should implement the specified behavior for.
So basically all the classes specified in the specification should be in the jar as contracts. End users of the jars can use them for the contracts. but there is no implementation. You need to have an actually implementation to run the application, though the spec API jar is enough to compile a complete JAX-RS compliant application.
For example, if we have one of those spec API jars on the classpath, we can author an entire JAX-RS application and compile it, but in order to run it, if we don't have the actual implementation, we need to deploy to a server that has the actual implementation of that spec version, for example JBoss or Glassfish
jaxrs-api - This is RESTeasy's packaging of the spec. It is not the official spec jar, but it does adhere to the spec contracts. RESTeasy uses this jar for the entire spec line, i.e. 1.x - current. Though the jar does change internals to adhere to the different JAX-RS versions.
jsr311-api - This is the official spec jar for the JAX-RS 1.x line.
javax.ws.rs-api - This is the official spec jar for the JAX-RS 2.x line.
jersey-core - This is a partial implementation of the spec. The rest of the implementation is contained inside other Jersey jars. Note that in earlier versions of Jersey, they actually packaged the JAX-RS spec APIs into this jar. It wasn't until later that Jersey started using the official spec jars.
jaxrs-ri - This is the complete Jersey 2.x implementation packaged into one jar. The "ri" mean reference implementation, which is what Jersey is: the JAX-RS reference implementation. If you are not using a dependency manager like Maven, you might want to just use this single jar instead of having to use all the separate jars that Jersey comes with.
Other Resources
Also note that though different implementation adhere to the spec, each implementation has its own set of extra features. To learn more you should go through the documentation of the different implementation. The three most popular implementations are Jersey, RESTeasy, and CXF