A lobbyist is a person paid to petition political representatives on a particular issue. Corporations, labor unions, and other special interests lobby Congress and federal agencies, hoping to secure favorable policies. Lobbyists might not be mentioned in the US Constitution, but those in the persuasion business wield extraordinary power.
So where does the influence come from? Special interest groups often endorse and raise money for candidates. Once a candidate is elected, the group (or lobby) calls on the official to oppose or support legislation. Lobbyists will educate representatives, circulating favorable policy research.
Special interest groups spend billions of dollars each year to retain lobbying firms located on Washington's infamous K Street. Other groups employ in-house lobbyists. OpenSecrets.org keeps a database of all the money spent on lobbyists. Just one example—since 1998, the pharmaceutical and health product industry has spent $1.2 billion on lobbyists.
Lobbyists also raise public attention on behalf of a cause, as opposed to a corporation or individual. For example, an organization committed to AIDS research can use collective action to draw attention to issues individuals would not raise on their own.
I think lobbying is primarily used by big companies that want something out of the government and tend to be more corrupt that helpful. I disliked Fred Thompson because he was a lobbyist during the 80's. McCain was named for taking tons of money from lobbyists during the 80's, one of the reasons I don't like him.
Lobbyists don't even go near Ron Paul's office, love that!