An User is a person from your team/company, that will have a specific access. Each user has a role & permissions, for example can be an Admin, a Manager, an Agent, Accountant…
A CRM is more adapted when you are working with others, because you can assign records (ex leads) and also each users can see all or limited records based on their role. For example, you are the manager of a team you want to see all leads, but each of your agent can see only their leads…