The network exists to help people communicate better. Content is the knowledge and information created by people. Content could be a Web page containing the text of a bill passed recently by the legislature, it could be an email, it could be a university professor teaching a class via the Internet to dozens of students located around the state. Services are the way content is delivered. The World Wide Web is a service that uses the Internet.
Email is another service.
Videoconferencing is a service. Each service requires a certain amount of computer hardware and software to work. Infrastructure includes cables and wires, network electronics, and the circuits and pathways that connect buildings and localities to the larger Internet. If we think of networks as a system of roads, then network infrastructure is the road system. Services are trucks carrying content to and from various locations.