I was asked this question on Quora.
First let me say that I am a registered Lobbyist but that is not my full time job. I take on select efforts that I feel are important; for example I helped push the JOBS ACT.
Lobbyist are simply people or organizations that attempt to influence law makers. A Lobbyist is required to become registered when they donate more than $5,000 to a politician.
There are many types of Lobbyist.
Labour organizations, trade organizations and Unions are the most common and lobby for laws that effect organization.
There are professional Lobbyist that will lobby for a effort for a fee.
While lobbying gets a bad name and some groups deserve the bad press; it is a necessary part of our political system.