There are two types of obligations that employers have to obey.
The first types of obligation are those agreed between employer and employee (which do not have to be in writing, although they often are). Usual terms that are agreed include salary and benefit levels, holiday rights, job duties and notice provisions.
The second types of obligation are those which are implied by Law. These include an obligation not to discriminate (on, for example, grounds of sex or race), an obligation to provide a safe system and place of work, and an obligation not to undermine trust and confidence.