Buckley emerged in an age in which Democrats and the New Deal coalition were dominant in America. This coalition would essentially give the Democrats 7 electoral wins out of 9 tries from FDR to LBJ with only Eisenhower in between.

There were various flavors of conservatism that opposed this coalition while it was around. These strands were not very well organized, but typically included ideas like pro-business policies, while also maintaining a preference for protectionism (there were strains of economic liberalism in these groups, but protectionism was typically more popular), social traditionalism, preference for small government, and non-interventionism.

Buckley, in his attempts to define and put boundaries on American conservatism, promoted staunch anti-communism (also present in pre-Buckley conservatism) that also included the acceptance of an internationally active America and a preference for economic liberalism and free trade. His efforts were to unite the various, disparate groups that opposed the New Deal, including libertarians, traditionalists, and anticommunists. This laid the foundation for the rise of Movement Conservatism that culminated in the election of Reagan and is still leading group in the Republican Party today. Two notes: (1) there were other streams added to movement conservatism over time including neoconservatives and the religious right, who took were sort of more extreme incarnations of anticommunists and traditionalists; (2) Trump and his ideological allies are pushing against many accepted tenets of Movement Conservatism.

Buckley, as he united certain groups, also attempted to keep out certain groups from conservatism who he saw as poisonous to the movement. This included Ayn Rand and her allies, the John Birch Society, anti-Semites, (eventually) segregationists and white supremacists, etc.

In all, prior to Buckley, there were a number of ideological streams that could be classified as conservative, including libertarians, traditionalists, and anti-communists, but they were not really united in any way. Buckley sought to unite these groups, while also keeping other groups out of mainstream American conservatism.

See for more:

The Conservative Intellectual Tradition since 1945 by Nash (published in 1976, revised in 1996)

William F Buckley Jr by Judis

As a follow-on, I've read that he said something along the lines of "We have kooks and they have kooks, but there are more kooks on our side". Did he actually say that, and if so to whom was he referring? Obviously John Birch Society, but any other organized groups?