EMZETT.Network Services
In short: Functions that a server or device provides to other participants over the network — e.g. name resolution (DNS), address assignment (DHCP) or file shares.
In more detail: Each network service usually runs on a fixed port and waits there for incoming requests (e.g. DNS on port 53, HTTP on port 80). The totality of active services on a system largely determines its attack surface — services that aren’t needed should be deactivated for security reasons.
In Depth
At its core, a network service is a server program that permanently “listens” on a particular port (in technical jargon: it’s in the “listening” state) and waits for incoming requests. Well-known examples with their standard ports: DNS (53), DHCP (67/68), HTTP (80), HTTPS (443), SSH (22), FTP (21). This assignment of services to fixed ports is a convention (managed by IANA), not a technical necessity — an administrator can in principle run a service on any free port.
What matters for security is the total set of all active services on a system, because every running service can potentially contain a vulnerability and thus offer a point of attack — this principle is called “minimising the attack surface” (attack surface reduction). A port scanner such as Nmap can make visible from outside which services on a system are reachable at all — an important first step both when securing your own systems and in a penetration test.