CompTIA, an IT association, has published a guide on DDoS (Distributed Denial of Service) attacks. This guide will help security professionals understand everything from the basics of detection to tools for combatting attacks, along with the skills one needs to prepare for cybersecurity incidents of this kind. This DDoS handbook is intended to act as a guide for everyone, from entry level to expert, and can be applied across industries.

Read it at https://www.comptia.org/content/guides/what-is-a-ddos-attack-how-it-works.