User: a Salesforce user who can log in. Users of the customer and partner portals are also included.
Account: a business account that typically represents a firm and typically has child contacts (i.e. people in the company)
Role: In an organisation, a role is a hierarchical group of users that is utilised as part of the sharing model. Users with higher roles have access to view records that belong to users with lower roles (assuming their Profile allows it). Users are not required to be assigned to a role.
Profile: a collection of permissions and access rights for a user including CRUD permissions. A user can only have one profile at a time. Permission Sets, which are a many-to-many relationship with users and augment profiles, are another something to consider.
Owner: A record's owner is a user or a group (queue). Who can see a record is controlled by record ownership, as well as the job hierarchy (and a few other factors).