Members of Congress can form several types of caucuses, but the most common types align along (1) party platform interests, (2) ethnic or racial concerns, (3) ideological similarities, or (4) the more ambiguously named "interest groups". This last type might be easily confused with the non-congressional advocacy groups that include lobbyists and/or regular citizens. Certain types of caucuses may have the support of both major parties from both Houses of Congress.
Examples on caucus types (corresponding to the numbers above):
(1). The Senate Democratic Caucus, formed in the 1870s, and formally organized in 1903.
(2). The Congressional Hispanic Caucus, formed in 1976.
(3). The Blue Dog Coalition, formed in 1995.
(4). The Congressional Motorcycle Safety Caucus, formed in 2009.
An official caucus is only recognized as such when it has been registered with the Committee on House Administration.
Congressional caucuses are formed whenever one or more members of Congress, usually in the House of Representatives, hope to form a coalition of like-minded representatives to better advance the common agenda. That agenda can be based on shared parochial issues, like when congressmen in whose districts components of a larger weapon system or ship are manufactured band together to support legislation to mandate federal agencies purchase those items. The caucus can be established to represent the common interests of geographic regions, like the former House Sunbelt Caucus, that was formed to represent the common interests of southern states, or a caucus can be established to represent the common interests of members of Congress from a certain, specified ethnicity, such as the Congressional Black Caucus, the Congressional Hispanic Caucus, and the American Sikh Congressional Caucus. They can be formed for the purpose of advancing an ideological agenda, as is the case with the House Progressive Caucus, whose members support more liberal and left-wing economic and social policies. There are caucuses comprised of members of Congress whose districts are dependent upon certain industries, as with the aforementioned military-oriented organizations, one of which is the Congressional Shipbuilding Caucus, whose members unite to support funding for navy shipbuilding programs.
In short, there is a long list of congressional caucuses established to advance any number of political agendas. The Congressional Invasive Species Caucus exists so that members of Congress whose districts border fresh-water lakes adversely affected by non-native species of fish and algae, thus threatening the future of those lakes and the industries, like fishing, that profit from them, can work together to address those issues. The bottom-line is that caucuses are formed to bring together elected representatives, often supported by the industries, ethnicities, or causes with which they are associated, to advance or protect their respective interests.