The Equality Act lists nine protected characteristics, which it is unlawful to discriminate against. In other words, you can’t not hire someone because of:
They are or are becoming a transsexual person
They are married or in a civil partnership
They are pregnant or having a child
They have a disability
Race including colour, nationality, ethnic or national origin
Religion, belief or lack of religion/belief
The protection extended by the Equality Act applies to the recruitment advertising process too – it is illegal to imply that you will discriminate against one of these characteristics by the language you use in any job adverts you place.
Terms such as ‘waiter’ or ‘waitress’ imply gender, and as such must not be used. Neither should any reference to age, and explicit, gender-specific terms like ‘PR girls’.
Aside from the obvious moral importance of upholding these standards, if you break the law and a potential candidate feels they have been discriminated against, they are quite within their rights to instigate legal proceedings against you.
This legislation obviously only applies to the UK, and different countries have different laws. But wherever you operate, upholding these standards as a matter of course will see you right, both legally and morally.