A traditional eight hour
day shift.
In some companies, the 9-5 is really 8:30-5 or 9-5:30. It's an extra half hour because of an unpaid
30 minute lunch break. In companies where you're paid
by the hour, it's so you don't lose out on 30 minutes of pay. In ones where you're paid by the year, it's pretty much squeazing the extra half hour out of you in order to make up for the 30 minute lunch.
It generally refers to working Monday through Friday but in modern times where services are expected to be availalble durring non business hours, evening and weekend shifts are becomming an increasingly required demand of employees.