Criminal defence barristers are responsible for defending people who are accused of committing a criminal offence. It's important to understand the difference between criminal claims and civil claims, and also the difference between a barrister and a lawyer.
Essentially, criminal defence barristers are a type of lawyer - they work at the higher levels of the courts, and they are specialists in litigation and in advocacy. They don't tend to operate in smaller, local courts, but rather in the superior courts. They don't work with clients in quite the same way that a solicitor does, but are higher level practitioners of law and advocacy.
There are only around 15,500 barristers that are currently practicing independently, and just ten percent of those are QC's. The others are junior barristers. Some are instructed by members of the public, who can access their services through the Public Access Scheme. Via this scheme, barristers can offer representation in court, and give legal advice to members of the public. Thy can represent their clients in any court or at any tribunal in the country, and guide the client through the procedure in question.
Barristers are not limited to the United Kingdom, although the QC title is one that is used only in England and Wales. Many other countries have barristers as well. The way that barristers work varies from country to country, and even in different parts of the commonwealth. Depending on the country that they are operating in, barristers may have different titles, legal roles, and obligations.
It takes a lot of study to become a barrister. There is a 'Bar Exam', and barristers must spend some time shadowing more experienced legal practitioners before they are allowed to take on their own tasks. Even when someone is qualified to operate as a barrister, there are limitations on the work that they can do. Only one in 20 barristers in the UK are currently qualified to do Public Access work, for example, and there are special schemes that barristers must go through before they can take on certain kinds of work for clients that are professionals.
A part of the reason that this kind of work is so rare is that it is a relatively new thing for barristers to be allowed to take on such work. The change in the rules did not occur until 2004, and prior to that barristers were limited in the work that they could do.