Sorry but I disagree with some of the classifications/comments in this thread so thought I would add to it.
In short,
Internships- also known as vacation work, typically refer to someone who is undertaking a temporary employment role with the aim of obtaining a graduate position and the employer similarly is looking for potential graduates. The internship is usually conducted over the summer holidays in Australia but can occur during the winter break, or even during semester depending on arrangements and scholarships (e.g. coop). At least in Commerce, internships are for penultimate students (i.e. if you are in a 3 year degree, you apply in your 2nd year and undertake the work over the summer break between 2nd and 3rd year). Internships can be paid or unpaid depending on industry but keep in mind you are still protected by relevant employment laws as well as the industry body- e.g. some bodies will not permit 'unpaid' work where you are clearly contributing/doing work and they are just not paying you.
Cadetships- these are very different to internships in that they are full time employment with a specific firm/company. Unlike the sometimes similarly advertised coop programs, a cadetship is literally a full time job with one company for a set period of time whilst you complete your undergraduate degree. As you are doing full time work, the employment contract typically requires that you only undertake part time studies during that time period. For example, a normal Commerce degree takes 3 years however if you do a cadetship, you will normally take one of those 3 years and do it on a part time basis (i.e. half load- 2 subjects per semester instead of 4) and stretch it out to two years when you are working full time. Cadetships being full time employment include a full year salary and you usually participate in all the other benefits/rewards of the company (e.g. mobile phone provided, laptops, subsidies, company vacations/events/parties, etc). Cadetships are also offered in a variety of industries but the most commonly discussed one on these forums is probably accounting. Others such as business services, construction, etc also exist.
Clerkships- these are similar to internships except they are for law firms and students in penultimate years. Unlike internships where companies use it as a way to trial staff and then take people on at graduate level both from the successful interns and fresh grads from uni, a lot of law firms have been known to only recruit for graduate roles through their clerkship programs.
Volunteering- unpaid work. Difficult to define beyond this but it's an option you have if you can't get any of the above and need some work experience on your CV.
