A user is a person with a password. An account is the commercial record. Keeping them apart is what lets one person manage several accounts without sharing a password.
Users
Users sign in to the portal. One user can be linked to several accounts, which is how an agency manages its clients, or a bookkeeper several businesses, with one login. The account owner invites further users and sets what each may do.
Contacts
A contact receives the kinds of mail you assign, billing, technical, support, and nothing else, and does not sign in. Use contacts for an accounts department that needs the invoices but not the portal.
One person, one login
The same identity can also be a member or a staff member. Each record lists the others it is linked to, so one person is never three strangers in your system.
Full guide
The complete reference is One person, one login in the WBAMS documentation.