What is a 'service'?

Microsoft Windows (good speak)

"a service is software that performs automated tasks, responds to hardware events, or listens for data requests from other software." (https://www.computerhope.com/jargon/s/service.htm)

"a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections." (https://en.wikipedia.org/wiki/Daemon_(computing))

Linux/Unix...

"We fancifully began to use the word daemon to describe background processes that worked tirelessly to perform system chores".[2] Unix systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background. (https://en.wikipedia.org/wiki/Daemon_(computing))

A service/daemon is the software that provides that service. The computer the service software is running on is called a server.

A Web Server runs a service program that listens on a TCP port(s) and responds to requests that are directed to that port(s). 80, 8080, 81, 8181, 8000, 8008...

Most current management tasks are provided through a web service that runs local commands based on HTML browser requests sent by an authorized user.

Web servers are often part of a 3-tier system that compromises most Internet activity. A person sends a request to a web server for information from a separate database server, action at the services computer, etc

Web services typically use the HTML markup language along with HTTP directives to communicate commands, details, and results.

Typically, a web server response consists of an HTML page with embedded JavaScript code. The browser that recieves that HTML page reviews it to determine how to render the response in the browser. It also identifies links to additional files (images, JavaScript modules, CSS sheets, etc) that are required to render the page. All of those additional files are then requested and incorporated into the rendering of the final browser screen.

File services simply provide disk space on which to store, and retrieve files. A file server presents the files it contains as being in a typical file system consisting of subdirectories and files.

It enforces access through permissions list that define what a user can do; including saving, reading files and directories.

A print service typically uses port 9100 to receive print job requests. This is where a client sends a file along with instructions for it to be printed. The file must be in a format that the printer understands, or that the client can translate into an understood format. The print drivers on the client help connect to the print service, and define the details of how a file is to be printed.

The print server sends messages back to the client informing them that the job has been received and put into a print queue, and then when the job has completed successfully.

A print server also sends error messages and alerts back to the client such as; our of paper, ink is low, paper is jammed, etc

Typically, a database server runs an Remote DataBase Management System (RDBMS) program such as MS SQL, MYSQL, mariadb, Oracle, etc.

An RDBMS manages any number of actual databases, with access being enforced for each separate database. Databases are composed of tables which are the file formats used to store the actual data.

A database also has triggers which are SQL scripts that run automatically when an event occures (such as a record being altered), sachems which are the data definitions layouts defining each column in a table.

An application server listens on one or more ports for code/encoded commands from other computer programs similar to, but different from Web Services. Application services are often proprietary, and rely on a compiled application on the client side which send commands to the service and reports the responses it receives from the service.

Rather than a port that the client simply establishes a session with, application services use Application Program Interfaces (APIs). If you query an API it will respond with a list of the commands it understands and any information it requires for each command. The client program can then format a command and any required information and send it to the API resulting in a response. The commands can trigger local processing, data exchange, and so on.

File Transfer Protocol (FTP) service simply store and share files. They are much more simplistic than a File Server.

Simple Mail Transfer Protocol (SMTP) is a service that provide email postoffice services.

A client logs into an SMTP server with a client program or browser and interacts with the SMTP system through the inbox and other folders used to store and manage email. The email can be stored on the server or client, or both.

Network Assessable Storage

Similar to a file server, NAS make disk space available to authorized users. But also tend to provide a suite of applications for managing and using files in different ways.