Megan's Law is the name for a federal law, and informal name for subsequent state laws, in the United States requiring law enforcement authorities to make information available to the public regarding registered sex offenders. Laws were created in response to the murder of Megan Kanka. Federal Megan's Law was enacted as a subsection of the Jacob Wetterling Crimes Against Children and Sexually Violent Offender Registration Act of 1994, which merely required sex offenders to register with local law enforcement. Since only few states required registration prior to Megan's death, the state level legislation to bring states in compliance —with both the registration requirement of Jacob Wetterling Act and community notification required by federal Megan's Law— were crafted simultaneously and are often referred as "Megan's Laws" of individual states. Thus, federal Megan's Law refers to community notification (making registry information public), whereas state level "Megan's Law" may refer to both sex offender registration and community notification.
Individual states decide what information will be made available and how it should be disseminated. For example, they disseminate the information via social media platforms such as Facebook. Commonly included information is the offender's name, picture, address, incarceration date, and offense of conviction. The information is often displayed on free public websites, but can be published in newspapers, distributed in pamphlets, or through various other means.
At the federal level, Megan's Law requires persons convicted of sex crimes against children to notify local law enforcement of any change of address or employment after release from custody (prison or psychiatric facility). The notification requirement may be imposed for a fixed period of time—usually at least ten years—or permanently. Some states may legislate registration for all sex crimes, even if no minors were involved. It is a felony in most jurisdictions to fail to register or fail to update information.
Together, Jacob Wetterling Act and Megan's Law provide two major information services: sex offender registry for law enforcement, and community notification for the public. The details of what is provided as part of sex offender registration and how community notification is handled vary from state to state, and in some states the required registration information and community notification protocols have changed many times since Megan's Law was passed. The Adam Walsh Child Protection and Safety Act supplements Megan's Law with new registration requirements and a three-tier system for classifying sex offenders according to certain listed offenses requiring registration.
This summary is from Wikipedia.