You should expect your department to make clear what it expects you to learn from your placement. This is usually a combination of discipline-specific knowledge and skills, alongside generic skills such as teamwork and project management. Your department will ensure that each placement they arrange or approve offers you adequate opportunities to achieve those expectations. Of course you are likely to enjoy a very wide range of experiences and develop less tangible characteristics as well, such as greater maturity and independence. You will want to be able to show potential employers and others what you learnt from your placement, in job applications and interviews or in your CV. Your department should help you understand how the placement fits in to your academic course and you should think about the contribution your placement can make in your personal development planning (PDP).
Your department will tell you whether they assess your placement separately, and if so, how. Assessment might be by means of a learning log or report, completed during and/or after your placement, perhaps supplemented by an oral presentation. Some departments choose to assess the skills and knowledge you will develop during your placement in later assessments connected to other modules. In any case, your placement is an integral part of your course and what you learn during it will have an impact on all your subsequent academic work.