Network services are services that run as a shared resource on the network server (or in some cases on a dedicated server). A lot are run by daemons so are transparent to the clients. Typical network services would be e-mail, printing, network file systems (such as virtual drives), password authentication, Domain name system (converting URLs into IP addresses), server-side virus detection, dynamic host configuration (allocating dynamic IP addresses), mail server spam detection, etc.
Basically all the day-to-day running and maintenance of the network.
