Lawyers, attorneys or barristers who represent persons facing criminal law charges.
In the US, given the predominance of the term "attorney", these lawyers are called "defense attorneys", whereas in common law jurisdictions, they are referred to as "defence counsel" or barrister.
A defense attorney or defence counsel is usually hired directly by the person accused and from that point forward, represents the accused throughout the legal process, including trial.
It is a fundamental principle of most justice systems that the accused person does not need to testify (see, for example, the US Fifth Amendment). Allowing an agent, such as a defense attorney to represent the accused in court, means that the accused does not speak or testify but still gets to cross examine the prosecution's witnesses. As an agent, the defense attorney brings is or her expertise to the task.
Sometimes the defense attorney is a public defender or otherwise paid by the state such as a "legal aid lawyer". In most jurisdictions, this means that the defense attorneys legal fees are paid for by the government but that in all other respects, the relationship between the defense attorney and the accused is the same as if the defense attorney was being paid directly by the accused.
Just about every courthouse has nearby several private law firms offering criminal defense attorney services. Any defense attorneys have experience as a prosecutor, which assists them in effectively defending their client from criminal charges.
Defense attorneys often take very unpopular cases and are asked "how can you defend that pig"? To this, the defense attorneys answer must be that everybody is entitled to a proper legal defense or that it is not a lawyer's job to determine guilt; that is left with the judge or jury.
Many lawyers are not able to act as defense attorneys as it does require, from time to time, the representation of clients from the lawyer personally dislikes. There are obvious motivational challenges for a defence attorney purporting to represent a repeat offender/child sexual abuser where his/her professional ethics demand that they "endeavour by all fair and honourable means to obtain for a client the benefit of any and every remedy and defence which is authorized by law".
References and Further Reading:
British Columbia Law Society Handbook of Professional Conduct, Chapter 1, ¶3(5), available as of Nov. 2007 at lawsociety.bc.ca/publications_forms/handbook/handbook_toc.html.