Sexual harassment is unwanted conduct of a sexual nature with the purpose or effect of violating your dignity or creating an intimidating, hostile, degrading, humiliating or offensive environment for you. This kind of conduct is prohibited by the Equality Act 2010.
Conduct of a sexual nature could include, for example, inappropriate remarks, touching, sexualised comments, or lewd or crude images such as offensive screensavers, Facebook posts or texts.
If you are experiencing this kind of behaviour, keep a diary of the remarks and incidents, your attempts to have the issue dealt with, and any changes in behaviour towards you as a result, for example being picked on by your manager, or suddenly being graded as 'unsatisfactory' when your work has always been regarded as 'satisfactory' in the past.
Keep a copy of any offensive texts, screen shots, postings and so on.
For conduct to amount to sexual harassment, it must be unwanted. Most sexualised attention at work is self-evidently 'unwanted'. Some colleagues may not realise that you find their behaviour offensive. However, certainly as soon as you have made your feelings clear, by explaining how their actions make you feel, there can be no question in anybody's mind that the behaviour is unwanted. As a result, any further continuation of the behaviour is likely to constitute unlawful sexual harassment.
This kind of behaviour at work is unacceptable and cannot be tolerated, but it is still prevalent in UK workplaces, to a shocking degree. In 2016, the TUC joined forces with the Everyday Sexism Project to produce a report - Still just a bit of banter? Among its findings, more than half of women - and nearly two-thirds of women aged 18-24 - reported experiencing workplace sexual harassment. 80% of incidents went unreported, for reasons such as fear of career damage, of not being believed, or of not being taken seriously.
Your employer has a duty to keep the workplace free of sexual harassment and will be liable unless it can prove that it took all reasonable steps to stop the harassment happening in the first place.