Knowledge management is the practice of capturing, structuring and sharing what your people know, so the right answer is available at the moment it is needed. In ITIL 4 it is one of the general management practices: it supports every other practice, from incident and request handling to change and problem management.
A knowledge base is not knowledge management. The knowledge base is the repository: articles, guides, procedures. Knowledge management adds the rules around it: who writes, who reviews, when an article is retired, how failed searches become new articles. Without those rules, a knowledge base becomes an archive nobody trusts.
Knowledge management is one of the practices in our practical guide to ITSM. It is also what makes self-service work: see service request management.
Knowledge reaches users through the portal: see the self-service portal in HaloITSM.