What is a lobbyist?

From Wikipedia:

Lobbying (also Lobby) is a form of advocacy with the intention of influencing decisions made by legislators and officials in the government by individuals, other legislators, constituents, or advocacy groups. A lobbyist is a person who tries to influence legislation on behalf of a special interest or a member of a lobby. Governments often define and regulate organized group lobbying that has become influential on policy.

Overview
In the United States the Internal Revenue Service makes a clear distinction for nonprofit organizations between lobbying and advocacy limiting the former to "asking policymakers to take a specific position on a specific piece of legislation, or that ask others to ask the same"; in common language, the definition of lobbying is normally broader. Other activities that seek to influence policies, possibly including public demonstrations and the filing of "friend of the court briefs", are termed as "advocacy".

The House of Commons Public Administration Select Committee argued that while there are shortcomings in the regulation of the lobbying industry in the United Kingdom, "The practice of lobbying in order to influence political decisions is a legitimate and necessary part of the democratic process. Individuals and organisations reasonably want to influence decisions that may affect them, those around them, and their environment. Government in turn needs access to the knowledge and views that lobbying can bring."

Governments often define and regulate organized group lobbying. Economist Thomas Sowell defends corporate lobbying as simply an example of a group having better knowledge of its interests than the people at large do of theirs.

Lobby groups may concentrate their efforts on the legislatures, where laws are created, but may also use the judicial branch to advance their causes. The National Association for the Advancement of Colored People, for example, filed suits in state and federal courts in the 1950s to challenge segregation laws. Their efforts resulted in the Supreme Court declaring such laws unconstitutional. At any given time, there are hundreds of cases in state and federal courts in which Advocacy groups are suing in hopes of winning lawsuits to help their members. Court victories, in addition to their legal benefits, can make the headlines and give interest groups a lot of publicity.

They may use a legal device known as amicus curiae, literally "friend of the court," briefs to try and influence court cases. Briefs are written documents filed with a court, typically by parties to a lawsuit. Amicus curiae briefs are briefs filed by people or groups who are not parties to a suit. These briefs are entered into the court records, and give additional background on the matter being decided upon. Advocacy groups use these briefs to both share their expertise and promote their positions.

United States
The ability of individuals, groups, and corporations to lobby the government is protected by the right to petition in the First Amendment to the United States Constitution. Lobbyists use time spent with legislators to explain the goals of the organizations they represent and the obstacles elected officials face when dealing with issues, to clients.

Lobbying activities are also performed at the state level, and lobbyists try to influence legislation in the state legislatures in each of the 50 states. At the local municipal level, some lobbying activities occur with city council members and county commissioners, especially in the larger cities and more populous counties.

Since 1998, 43 percent of the 198 members of Congress who left government to join the private sector have registered to lobby using the 'revolving door of influence'. The Lobbying Disclosure Act of 1995 and Honest Leadership and Open Government Act of 2007 increased regulation and transparency. In 2009 U.S. President Barack Obama signed two related executive orders and three presidential memoranda on his first day in office with the goal of making his administration more open, transparent, and accountable.