The law on equality talks about two types of discrimination.
The law covers certain cases where what seems like discrimination is allowed. For example, it is not against discrimination laws to insist that a counsellor working in a women's refuge should be a woman. And it might be legal to say that a particular job was not suitable for a person with a disability for safety reasons.
If you want to complain about being discriminated against (or if you are helping a colleague who is complaining), you may fear that your employer may treat you less favourably for this reason. If they do, this is called victimisation, and it is unlawful (illegal) in the same way discrimination is.