What's said above is true. but often times overtime is paid extra in a salaried job.
So for example, my job is advertised as a salaried job.
I am paid as a salaried wage.
that is to say, that I get the same amount this month (20 working days of february) as I did last month, 23 working days.
despite having worked some 24 hours longer. and I'll get paid the same next month when there are 21 working days.
obviously if I were paid hourly I'd be getting 23 x 8 lots of hours in January, (big pay check) and 20 x8 hours in February, (smaller pay check).
when you are salaried your wage does not alter with your hours worked in accordance with your job description.
which is where it gets confusing.
I used to work in a burger king when I was a student. on bank holidays, people like me (hourly rate staff) were paid double time. and given time off in lieu, (which was actually just paid so we got tripple time). when I worked bank holidays my hourly rate was higher than the rate applied to managers, (e.g we earned more for working that Monday than the manager did, because the manager was salaried.)
Now the hourly rate was set as a perk to make sure that staff were willing to work on the bank holiday, such that there would be adequate levels of staff to meet demand.
the managers were paid less, but only for that day, ordinarily they were paid more, it just sucked for them on that day.
in that case even though it's a holiday, the business is still open, it's a Monday, and the manager that works Mondays still has to work Mondays, because that's what his contract says.
However, if you are a person who works 9 - 5, (or 9:30 - 6 as I do). and you are asked to come in early, asked to stay late, or asked to do work out of hours etc, then that is outside of your normal duties as defined in your contract, and you are paid overtime.
Basically, you agree a contract, and agree a salaried rate for that contract, being asked to do anything outside of that contract is open to negotiation of pay, or additional benefits, (for example I've worked overtime into an evening not for overtime, but on the understanding that I've earned time off in Lieu and therefore accrued extra holiday days.
Most contracts include clauses regarding reason ability, staying 15 minutes late occasionally to finish your work is reasonable, you would not expect additional money in your salary to account for this.
however, if you stay 15 minutes every day over a 20 days working month that's an extra 5 hours, (nearly an extra day) after a while this may fail to be a reasonable request (that you stay late every day without additional numeration) and so you would be justified in either asking that you be paid for your services as overtime (since it happens every day), or that you need additional staff members to complete work set.