A service is, essentially, a function offered by a server. In client-server terminology, a host that initiates a request is a client, where a host that responds to a request (and performs some operation) is a server.
Most hosts, whether they are on an intranet of the Internet, use TCP/IP (tranport control protocol/Internet protocol) to communicate with another host. This means that, in theory, any service can be intranet based or Internet based.
This section explores some of the more common TCP/IP services, and analyze their intranet and Internet suitability.