Short version: OTRS helpdesk is the core technology of recording and allowing agents to respond to requests. OTRS ITSM imposes additional IT service management tools and concepts on the processing of requests to help you implement a ITIL-based process customized to your organization. ITSM is not ITIL-in-a-box (you won’t be magically ITIL-compliant if you implement it). Think of it as a toolkit to use in implementing your own ITIL-compliant processes.