@ich777Sure, I´ll help explain.
Server: Zabbix server is the central process of Zabbix software.
The server performs the polling and trapping of data, it calculates triggers, sends notifications to users. It is the central component to which Zabbix agents and proxies report data on availability and integrity of systems. The server can itself remotely check networked services (such as web servers and mail servers) using simple service checks.
Web: Zabbix web interface is a part of Zabbix software. It is used to manage resources under monitoring and view monitoring statistics.
Agent: Zabbix agent is deployed on a monitoring target to actively monitor local resources and applications (hard drives, memory, processor statistics etc).
So all three is a part of getting it up´n running.