A client is something or someone who is requesting some resources and the server is basically a machine which serves the client request.
For example, suppose you want to check your mail, then you go to the website yahoomail.com. Here you are nothing but the client to the yahoo server. Yahoo server is a web server or http server. Web server is a machine which can take request from client and can give back the response, and basically that response is static response page or any page or any file that is static in nature. So when you logon yahoomail.com, it sends a request to the yahoo server across the web and this request is nothing but http request. Http is a protocol stands for hyper text transfer protocol and everything you communicate over web should be http. So this web server can understand only http, nothing else. Then the server checks the page that you requested in the repository. So from its repository it picks one static page from bunch of pages and sends that page across to the client over the web again, and this response is called as http response. Here client is nothing but your web browser whatever you are using like internet explorer, Mozilla fire fox, etc..which are technically as client to the web server.