A server is a software that listens to clients’ requests from the Internet and responds accordingly.
Sometimes a server can also be a client. For example many websites display Google Maps on their webpages. They did it by requesting Google Maps pages from Google’s server and put it into webpages. Thus to Google’s server these websites are clients, while to their website viewers they are servers.
- Restaurants: they serve food and drink for customers’ requests. On the other hand they are clients to meat/vegetable suppliers, requesting ingredients in order to make cooked food.