Commerce/Business is basically the studies relating to how companies operate whether it be their strategies for targeting customers (marketing), how they do their book keeping/reporting and make decisions (accounting), how they raise funds to start off the business, where they can get money from or how they choose to invest their money elsewhere (finance), etc... it's a very broad subject in the sense that it can be applied not just to companies but also to yourself.
Since you're studying economics and legal studies, you're kindof on track. Economics is used to identify how the economy impacts on businesses (at the most basic level, things like supply/demand affect how much stuff a company could sell for example), and the legal side is that there are a lot of legal implications and issues for businesses (the first one I think they teach is the different structures a business can take- e.g. Sole traders, Private company, etc... similar to Year9/10 Commerce).
Postgrads can do anything- Commerce can be applied to any industry really.
Job opportunities- depends on your specialisation, but you can't go wrong with Accounting
Pay- ask other peope lol
Hours- depends on the company
Law- always helps in terms of getting the job but for most Commerce jobs, law isn't that useful (in terms of when you start working) and it's just a waste of time if you don't plan on utilising it IMO.
ps. Others will probably disagree with my very much oversimplification of the topics mentioned in this post, but I thought I'd do my best to write in 'simple terms'.