What is the difference between a lobbying group and a PAC?

tl;dr: PACs hire lobbyists. A lobbyist is a professional whose job is to make contacts with influential people in Washington (or whatever government) and make a case on behalf of a client. They're regulated under the Lobbying Disclosure Act of 1995. read more

A Political Action Committee is a group of people with some kind of interest. They collect money and spend it to promote that interest. They have to file forms, with the Federal Election Commission rather than with the legislative branch, though unlike the lobbyists they have ways to not disclose who's giving them money. read more

campaign in which a group get its membership to contact government officials in support of the group's position. Political Action Committee (PAC) a private group that raises and distributes funds for use in election campaigns. read more