Here is a pretty good description from the Wikipedia page:
Caucuses in the United States
In U.S. politics and government, caucus has several distinct but interrelated meanings.
One is for a meeting of members of a political party or subgroup to coordinate members' actions, choose group policy, or nominate candidates for various offices.
The term is frequently used to discuss the caucuses used by some states to select presidential nominees, such as the Iowa caucuses. Along these same lines, in early American history, the Congressional nominating caucus and legislative caucus were influential meetings of congressmen to decide the party's nominee for President and party platforms. Similar caucuses were held by the parties at state level.
The other main context in which the term is used is for a subgrouping and of elected officials that meet on the basis of shared affinities or ethnicities, usually to affect policy. At the highest level, in Congress and many state legislatures, Democratic and Republican members organize themselves into a caucus (occasionally called a "conference"). There can be smaller caucuses in a legislative body, including those which are bipartisan or even bicameral in nature. Of the many Congressional caucuses, one of the best-known is the Congressional Black Caucus, a group of African-American members of Congress. Another prominent example is the Congressional Hispanic Caucus, whose members voice and advance issues affecting Hispanics in the United States, including Puerto Rico. In a different vein, the Congressional Internet Caucus is a bipartisan group of Members who wish to promote the growth and advancement of the Internet. Other congressional caucuses such as the Out of Iraq Caucus, are openly organized tendencies or political factions (within the House Democratic Caucus, in this case), and strive to achieve political goals, similar to a European "platform", but generally organized around a single issue.
Among American left-wing groups, a caucus may be an openly organized tendency or political faction within the group, equivalent to a European "platform." Examples would include the "Debs," "Coalition" and "Unity" Caucuses of the Socialist Party of America in its last years.
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