There are two basic forms of discrimination. Indirect and direct.

Direct discrimination is when an employee or prospective employee is less favourably treated because of their race or sex.

This tends to be obvious discrimination, for example, a female candidate with the best qualifications and experience does not get an interview.

Or, if she is already in a job, she is ignored for promotion, the job going to a less qualified male worker.

An employer cannot argue that it was not their intention to discriminate, the law only considers the end effect.
In-direct discrimination is a more complicated form of discrimination.
An Employment Tribunal will look at 3 factors.

1. The amount of people from a racial group or of one sex that can meet the job criteria is considerably smaller than the rest of the population.

2. The criteria cannot actually be justified by the employer as a real requirement of the job. So a candidate who cannot meet the criteria could still do the job as well as anyone else.

3. Because the person cannot comply with these criteria they have actually suffered in some way because of it. This seems obvious, but a person cannot complain unless they have lost out in some way.

With Indirect Discrimination an employer can argue that there may be discrimination, but that it is actually required for the job.

This does not happen very often, but circumstances where it might occur are, for example, actors who are needed to play certain characters for authenticity. The same can be true for restaurants, for example an Indian restaurant will want Indian staff rather than white staff.