A lobbyist is an activist who tries to persuade members of the government to enact certain legislation, usually that which would benefit their group. Lobbying is a regulated industry and a protected activity under the First Amendment of the Constitution.
Lobbying can also include activities such as researching and analyzing legislation, attending congressional hearings, educating government officials on important issues, working on advertising campaigns and influencing opinion "leaders."
Lobbyists must register with the government and report their activities annually in order to provide transparency on how lobbyists and government officials interact. This is done to ensure that corrupt practices can be tracked and taxed appropriately.