New User vs. New Login

[pwills]

We have a web-based client database system. Our office admin wants to do a mail-merge via the ODBC connection straight into the database. I want to create an account for the ODBC connection on her machine which I can lock down. I am somewhat confused between logins and users.

Question:
1. Should I create a new user or login?
2. What sort of role would the mail merge operator need?

Thanks in advance.