Internet Servers and Clients
Every machine on the internet is either a server or a client. The machines that provide services to other machines are servers. And the machines that are used to connect to those services are clients. There are web servers, email servers, FTP servers and so on serving the needs of internet users all over the world.
When you connect to www.howstuffworks.com to read a page, you are a user sitting at a client's machine. You are accessing the HowStuffWorks web server. The server machine finds the page you requested and sends it to you. Clients that come to a server machine do so with a specific intent, so clients direct their requests to a specific software server running on the server machine. For example, if you are running a web browser on your machine, it attempts to talk to the web server on the server machine, not the email server.
A server has a static IP address that does not change. A home machine that is dialing up through a modem, on the other hand, typically has an IP address assigned by the ISP every time you log on. That IP address is unique for your session — it probably will be different the next time you dial in. This way, an ISP only needs one IP address for each device, rather than one for each customer.