Impossible to say without knowing the company/industry/role I'm afraid, trainee isn't a term which has any legal meaning and means different things in different places. In general I would say a trainee job is usually for a fixed period of between 6 and 24 months (depending on how complicated the training is and whether any formal qualifications have to be obtained) and usually paid less than the job you are being trained to do, either by being at a lower grade or paid a % of the substantive salary (it still needs to be at least NMW though unless it's a proper apprenticeship).
What happens at the end of the training period varies, sometimes you have to apply and be interviewed for a substantive/non trainee role, sometimes you are guaranteed to move across if you successfully complete training. Usually it only makes sense for the company to invest in training you if there is at least a high chance of you moving into a substantive role afterwards , although I have heard of some unscrupulous companies using bogus 'training' positions simply as an excuse to pay less than the industry standard.
It would be sensible to ask at the interview or afterwards if you are offered the job what the training actually entails, is it all on the job or will you get any formal qualifications or certification, how long it will take and what happens once the 'traineeship' is over as only the company can tell you really. Good luck!