As a lawyer, I can answer that question pretty well. "Bar" has a couple of meanings. "The bar" can refer to all the lawyers in a particular area or a particular area of law. Examples: The California bar. The intellectual property bar. The intellectual property bar in Los Angeles. "Bar" also refers to the test a person must pass to be a lawyer in the United States, called the bar exam. If you getting a passing score on the bar exam, it is called passing the bar. The reason we use the word "bar" for these purposes is (I believe) that in older times, there was a small gate in a courtroom between where the audience sat and where the lawyers worked; this gate was called the bar. To be allowed to "pass the bar" and go to the lawyer's area, you needed to be a lawyer. I don't know if this is a true story, but I've heard it. "Law" has a number of meanings. Most generally, it refers broadly to everything having to do with the legal system -- the government that enacts laws, the courts that interpret laws, police that enforce laws, lawyers that help clients with legal problems, etc. Example: Many people in the United States are involved in the law in some way. "Law" can also mean all the statutes and rules enacted by government. Example: Don't do that; it is against the law. Or, intellectual property law in China is developing fast. "Law" can also mean a specific statute. Example: the government passed a law requiring motorcyclists to wear helmets. I hope this helps.

I don't really understand what you mean. You may need to explain a bit better (e.g. an example) of where you think they are equivalent. 'Bar' can be several things, but none of them are really directly equivalent to 'law'. It can refer to the profession of being a lawyer. 'He is a member of the bar'. It can be a ban or barrier to prevent people entering something. But legally, that may not have much if any standing, and may be enforced by something like a trespass order. It can apparently be 'a plea suspending an action or claim in a lawsuit.' (google dictionary) - but that sounds like a legal jargon term, it probably has quite limited scope, and would probably not be understood like that by anyone but a lawyer.

One legal meaning of "bar" in England is a section of the legal profession, whose advocates are called "barristers" . They are instructed by "solicitors" (another part of the legal profession) and ultimately by the client. They are essentially the people you often see in wigs and gowns in the UK's higher courts. Here is a link on their work: http://www.barcouncil.org.uk/about-the-bar/what-is-the-bar/ and another link on their history: http://www.barcouncil.org.uk/about-the-bar/what-is-the-bar/history-of-the-bar/ . Another meaning is the verb: to bar. This means you stop someone from doing something. If someone gets drunk in a pub, the landlord might "bar" them from the pub, which means they are no longer allowed in. Alternatively if someone is too young to drink in a pub, then they are not allowed in by "law". So a law is an act of parliament which has been voted upon by Members of Parliament. A bar is a restriction on someone doing something, which might or might not be based on an act.

What is the difference in usage when they can be understood as law?

In what context? Bar can be a couple of different things.