Under most circumstances it is not illegal to lie. If lying about your age or weight, for example, were a crime there would be far more people in jail than there currently are. Lies that are against the law are typically those that involve financial or legal matters. For example, lying to a judge, jury or police officer or lying in order to persuade someone to give up money can lead to serious legal problems.
Fraud is typically defined as one person, with knowledge of falsity, tells a second person something to induce them to act. It becomes a crime when the second person suffers injury or damage as a result of believing the false information. Fraud is most commonly associated with financial fraud, such as selling faulty products or worthless investments. It may also involve obtaining a job, a contract or some other benefit based on false pretences but only if another party suffered injury or damage as a result.
Perjury typically relates to deliberate lies told as part of a legal proceeding or other official proceeding that have a bearing on the outcome of that proceeding. False statements amounting to perjury, in most cases, are made under oath or affirmation to an official of the court, which may include the court clerk or a notary public. Some legal forms may also include the words "I swear or affirm, under penalty of perjury ..." or similar words, which would make false or misleading statements on those documents subject to perjury prosecution.
Libel and Slander
Libel or slander involve lies told that damage the reputation of another person or organisation. The primary difference between the two is the means of transmission. Libel occurs when the lies are printed or published, slander occurs when the lies are spoken. Any individual who wants to sue for libel or slander must show that the other party made the false statements and that the statements were made to at least one other person beyond the victim and the defendant. The plaintiff must also show that the statements in some way damaged their reputation.
Lying to Police
State governments treat lying to the police in a variety of ways. At the federal level, it is covered under U.S. Code Title 18, Part I, chapter 47, § 1001 "Statements or Entries Generally." This section makes it a crime to make any false or misleading statement or withhold or conceal a material fact to any officer of the federal government on any matter that falls under the jurisdiction of the judicial, executive or legislative branch of the government.