Short answers, pulled from the story.
The General Theory of Employment, Interest and Money was published in February 1936. Keynes had finished the manuscript in December 1935 and set the cover price at five shillings to encourage wide circulation.
The central argument is that the level of employment is determined by the level of aggregate demand, not by the price of labor. Keynes denied that competitive markets in equilibrium would automatically produce full employment, and argued that if total demand falls short of full-employment output, the economy must contract until the two are equal.
Say's law, as Keynes summarized it on page 18, holds that "supply creates its own demand," meaning wages paid for production are inevitably spent back into the economy, making a general glut impossible. Keynes rejected this by arguing that saving decisions and investment decisions are driven by different factors, and that the economy can settle in equilibrium below full employment.
In Chapter 12, Keynes used animal spirits to describe the spontaneous urge to positive action that drives most investment decisions rather than calculated mathematical expectation. He argued that when animal spirits falter and spontaneous optimism fails, enterprise fades and the economy contracts.
The Cambridge Circus, a discussion group that formed after the Treatise on Money was published in 1930, fed ideas to Keynes through Richard Kahn. After the Circus disbanded in May 1931, Kahn, Joan Robinson, and Austin Robinson continued meeting in Cambridge and produced a Manifesto in 1932 whose ideas Keynes incorporated. During 1934 and 1935 Keynes also circulated drafts to Kahn, Joan Robinson, and Roy Harrod.
The multiplier, introduced in Chapter 10, measures how much total employment is generated by a given injection of spending. With a marginal propensity to consume of 90 percent, Keynes calculated a multiplier of 10, meaning public works would produce ten times as much total employment as the works themselves directly created. Keynes built on a related concept introduced by R. F. Kahn in 1931.