There are many different trade unions, simply called
Facket in Swedish. They are linked to different industries so as to be better able to help their members. Being a member of a trade union can be a good idea in several ways. You have someone who can help you if you have problems at your workplace. Instead of having to monitor your own rights, negotiate your own wage, make sure by yourself that dangerous workplaces are made safer and doing something by yourself if you are harassed, for example, you then have an organisation behind you that can help you out in all these situations. A trade union can offer you expert assistance in many different areas, everything from safety issues to legal matters. A trade union also has more chance of having demands accepted since they are backed up by a whole organisation, while it is much easier for an employer to disregard demands and wishes that come from individuals.
Read more on LO's website and TCO's website.