A repeal is an abolition, also sometimes called an abrogation, of a law. Repeals most often occur because changing social norms make a law appear to be unreasonable or questionable. Sometimes they happen in response to agitation by members of society who lobby for a repeal of a law which they feel is unreasonable, and in other cases, lawmakers may independently decide on a repeal. Legislative bodies actually regularly quietly repeal laws without attracting very much public attention.
In an express repeal, a law is passed to specifically abrogate a previous law. An example of this is the 21st Amendment to the United States Constitution, which repealed the infamous 18th Amendment. By contrast, in an implied repeal, a new law is passed which conflicts with an old law and the new law takes precedence, effectively voiding the old law even though it may remain on the books.
In some nations, people have agitated for removal of outdated laws which are not enforced, or which have been superceded by other laws in implied repeals. Many people are surprised to learn that there are a number of peculiar laws on the books in their area, banning all manners of activity from walking snakes on Sundays to wearing pants while female. Some of these weird laws are collected in compendiums for general amusement, but they are never actually enforced, and repealers argue that while they are historical curiosities, they should still be stricken from the legal books.
In other cases, people who want to repeal laws want to do so because they believe that a law has very real and harmful effects. Many nations have repealed laws which institutionalize racism, for example, as social norms have changed and there has been a greater push for equal rights. Likewise, repealers have focused their efforts on laws which discriminate against other groups, such as women, gays and lesbians, and religious minorities, with the goal of making society more free and equal.
The process for repealing a law varies by nation. In some areas, citizens can use the initiative and referendum system to hold a vote calling for a repeal. In others, citizens must urge lawmakers to sponsor a repeal, and once the repeal reaches a legislative body, they can push other lawmakers to support it. Legislators tend to be especially amendable to suggestions in election years, when they are actively working to retain votes from constituents.