Matthew L.’s Answer
The law is a profession like medicine, dentistry, nursing or accounting and in order to practice law you need a state license. The license is issued by the state in which you practice. The licensing process is generally administered by an organization called the "state bar association". The bar association administers a test to every prospective lawyer called the "bar exam" which he or she must past with a certain score before being granted a license to practice law. Sitting for the bar exam generally takes two days. One day consists of a multiple choice test covering various general areas of the law, like torts, contracts, property law, criminal law, evidence and constitutional law. This section is the same for every state. The second day generally consists of a series of essay questions that cover topics that are more state specific. It usually takes a few months for the exams to be graded. If you fail, you can usually appeal your results or you can retake the exam.
In one state (Wisconsin) you do not have to take the bar exam to be admitted to practice if you graduate from a Wisconsin law school that is accredited by the American Bar Association (ABA). In most other states you need to graduate from law school before you sit for the bar exam. However, in California, Vermont, Virginia, Washington, and Wyoming you are also able to sit for the bar exam without first attending law school. This is called "reading the law" and if you can absorb all the law to pass the bar exam just from reading books, you can become a lawyer without going to law school. This is very difficult, takes a long time (usually years) and is not recommended. Most states also have what is called a "character and fitness" requirement meaning that the state bar also looks at your criminal history and ethical fitness to practice before granting a license.
Once you pass the bar exam for your state and the character and fitness review, you must pay an annual license fee of a few hundred dollars to maintain your law license. Some states also have a "continuing legal education" (CLE) requirement where you must complete classes to stay current in your proficiency to practice. Your license to practice may be revoked or suspended if you do not pay the annual fee, fail to keep up on your CLEs, or, in most cases, if you are convicted of a felony or you violate the state code of attorney professional conduct.
If you attempt to give legal advice to people or try represent someone in court without a law license, this is called the "unauthorized practice of law" which is a crime. Generally you are only licensed to practice in the state(s) where you pay your bar dues. You can be licensed simultaneously in multiple states but you will generally have to pay bar dues in each state and, in some cases, actually take the bar exam in those other states to get your license.