ISP: Internet Service Provider. ISP is the middle man between the client and servers. For the typical homeowner, the ISP is usually a “cable company.” When your browser gets a request from you to go to www.github.com, it doesn’t know where to look for www.github.com. So it’s the ISP’s job to do a DNS (Domain Name System) lookup to ask what IP address the site you’re trying to visit is configured to.
Client: An application, such as Chrome or Firefox, that runs on a computer and is connected to the internet. Its primary role is to take user interactions and translate them into requests to another computer called a web server. Although we typically use a browser to access the web, you can think of your whole computer as the “Client” piece of the client-server model. Every client computer has a unique address called an IP address that other computers can use to identify it.