>It seems that in the U.S., salaries are often expressed per year. For short-term work, hourly
>rates are sometimes used.

Briefly, Salaries in the US are what is paid by an employer to an employee. Salaries can be based on an annual amount (say $50,000 per year) and this is normally considered an 'exempt' employee (exempt meaning that the employer-employee relationship is not bound by labor laws governing pay). Salaries can also be based on an hourly rate (say $25 per hour) and this is normally considered an 'non-exempt' employee (employer-employee relationship is bound by labor laws governing pay).

Exempt employees are paid the agreed salary amount regardless of time spent (paid for work performance) and the employer is not bound to pay for overtime; non-exempt employees are paid only based on time spent (not based on performance) and the employer is bound to pay overtime.

Many companies have tried to 'bend' the rules or get around them in trying to require exempt employees to record minimum time and will dock if not met. I know of a number of cases where the US Labor dept sued these companies and they had to compensate the exempt employees and pay fines to gov't.