What is an exporter, and what is "scraping"?

An exporter is a program on the target system that reads values (CPU, RAM, a database's internals) and serves them as metrics over an HTTP API. Scraping is Prometheus fetching that endpoint.

This is the biggest mental shift from classic monitoring. Nagios-style checks are scripts that run, look at something and return a status code. Prometheus doesn't run checks at all. It expects every target to offer a REST endpoint full of metrics, and it simply reads that endpoint at a fixed interval.

• Node Exporter ships with the Prometheus project and covers the basics of a Linux host: processes, RAM, load, network traffic per interface, disk fill levels. It roughly matches the standard set of checks a Nagios admin sets up first.
• Other exporters exist for MySQL, HAProxy, MongoDB, IPMI and many more, many of them written by the community, because writing one is not hard.
• No exporter needed when the software already speaks Prometheus. Ceph and Docker, for example, expose a Prometheus metrics endpoint natively. Check the Prometheus wiki before installing an exporter.

On each scrape, Prometheus always fetches all metrics an exporter offers.

Go deeper:

• Prometheus — Exporters and integrations — the list of official and community exporters, and software that exposes metrics natively.
• Prometheus guide — Monitoring Linux host metrics with the Node Exporter — install it, curl /metrics, add it as a target.