The history of computing started off with centralised computers (in many cases mainframes) or servers performing all the calculations. Client computers were then attached to these centralised computers (servers) and if you wanted to calculate something, you would have to wait for the central computer to respond. As computing power got cheaper client nodes became more powerful and the central computer less important. However, with the growth of the internet, there has been a shift back to a client server model. Powerful central computers store information such as emails, documents, music and videos or offer services such as file hosting, printing, game hosting and internet access; client computers fetch information and use services from these central servers. In the next few years you are likely to see more and more software moving away from running on your desktop to running on remote servers and you accessing it as a client, this is called software as a service.