(FORTUNE Magazine) – The entertainment business has always been a riotous spectacle, but even interested observers have found it perplexing in recent years as debt-burdened players on both coasts spent billions for record companies, theme parks, cable systems, online services, incompetent executives, has-been pop singers, and high-megaton bombs at the box office. Did the moguls in Hollywood and New York have a real plan? Or were they perhaps ad-libbing strategies for the benefit of Wall Street?

The key to success in the media business was perhaps never better expressed than by Preston Beckman, an NBC senior vice president, who said in a newspaper interview last year: "I don't want to sound cheap or anything, but this business is about buying into success and reducing the cost of failure."

That's precisely what Disney is doing in its $19 billion merger with Capital Cities/ABC--and precisely what Westinghouse is not doing in its $5.4 billion deal with CBS. These two deals, made possible by loosening federal regulations, have taken an image of the media business long obscured by static and snapped it into focus. Amid quickening consolidation and growing heaps of debt, four conglomerates have emerged as dominant: Walt Disney, Viacom, News Corp., and Time Warner. On the pages that follow, FORTUNE profiles their leaders and, on page 86, their holdings. An explanation of the massive regulatory changes the moguls are exploiting begins on page 83, followed by a time line of the media business from Gutenberg to Katzenberg on page 84.