Now I've searched what is the meaning of REST server and REST client but I could not find a good answer. Could someone explain them?
When in doubt, go to the source: Fielding's dissertation where he defines the REST architectural style.
In Chapter 5, he describes Client-Server as the first of the constraints.
Separation of concerns is the principle behind the client-server constraints. By separating the user interface concerns from the data storage concerns, we improve the portability of the user interface across multiple platforms and improve scalability by simplifying the server components. Perhaps most significant to the Web, however, is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.
The description of the Client-Server architectural style is in Chapter 3, which Fielding attributes to Gregory Andrews
A client is a triggering process; a server is a reactive process. Clients make requests that trigger reactions from servers. Thus, a client initiates activity at times of its choosing; it often then delays until its request has been serviced. On the other hand, a server waits for requests to be made and then reacts to them. A server is usually a non-terminating process and often provides service to more than one client.
In short, a REST client and a REST server are both connectors
The essential difference between the two is that a client initiates communication by making a request, whereas a server listens for connections and responds to requests in order to supply access to its services. A component may include both client and server connectors.
Note that Fielding is being somewhat precise in his definition here; he uses the term components to describe the roles that are played.
A user agent uses a client connector to initiate a request and becomes the ultimate recipient of the response. The most common example is a Web browser....
An origin server uses a server connector to govern the namespace for a requested resource. It is the definitive source for representations of its resources and must be the ultimate recipient of any request that intends to modify the value of its resources....
Intermediary components act as both a client and a server....
In common discourse, I think you'll find that most understand REST client to mean the user agent component, and REST server to mean the origin server component.
