"Users" are LUSID accounts. with an account for each person with some access to LUSID. Users are allocated to Roles via LUSID's role based access control system. A series of Roles are created that model the job functions and data access rights in your organisation. Once these roles are configured they are assigned to the appropriate users. One user can be assigned multiple roles.
A service account is a special user account that an application or service uses to interact with LUSID. Visit this article to learn more.