A criminal defense lawyer is a lawyer specializing in the defense of individuals and companies charged with criminal conduct. In the United States, criminal defense lawyers deal with the issues surrounding the apprehension, searches of client or property, and arrest of his or her client (4th Amendment), as well as any statements the client may have made (5th Amendment). Criminal defense lawyers also deal with the substantive issues of the crimes his or her clients are charged with. In the United States criminal defendants are entitled to the presumption of innocence until prosecutors prove each essential element of a crime beyond a reasonable doubt. Serious crimes (e.g. felonies) in the United States are tried to juries of twelve people and the jury must be unanimous in its verdict to either convict or acquit the defendant. A split in the jury is often called a "hung jury" and may result in a retrial of the defendant. Criminal defense lawyers actively pursue their client's cause through all stages of a criminal prosecution.
Many criminal defense lawyers in the United States are employed by governmental entities (States, Counties, etc) and are often referred to as "Public Defenders." Often Public Defenders are fresh law school graduates seeking to gain quick courtroom experience, but there are many older, extremely well experienced lawyers who have made public defending a lifetime advocation. There are also private defense lawyers who are retained by individual clients on a case by case basis.
Criminal defense work can be intimidating to some lawyers as the specter of a client going to jail for long periods of time or even being subjected to capital punishment looms over some defendants. Because of this and other factors, criminal defense lawyers tend to be a special breed of lawyers.