Both work placement and work experience involve the generosity of a business to host a student or course participant for a nominated period of time (usually one week or one day per week for a defined period). And, it is fair to say that both provide the participant with the opportunity to become involved in a workplace and learn.
Work Experience (which is usually done in Year 10) is a less formal structure designed to introduce a student to a potential career, whereas in Work Placement students are actually studying your industry as part of their HSC. Work placement students are seeking the opportunity to apply their developing industry related skills as learnt in the classroom. This mean students turn up on the first day with some skills they have already learnt from the classroom!