Please note that this article mainly discusses the modern American media landscape.
The Modern Media Landscape 
The modern media industry (since about 1950 on)has been characterized by two paradoxical trends.
Media have trended toward conglomeration: One giant company owns several properties like movie studios, publishing houses, newspapers, radio stations, television stations and networks, and websites.
Media have also trended toward fragmentation: Where there were once four major television stations there are now hundreds, and while there used to be just a handful of major general interest magazines, there are now nearly a hundred in the same category. Not only that, but the number of categories has exploded. There are enthusiast magazines, websites, and TV stations that cater to every possible topic under the sun.
Only a few large corporations own a disproportionate amount of media and entertainment outlets. This allows for cross-promotion and larger budgets, but critics think it can limit creativity and independent media.
Instead of having just three TV networks like in the 1950s and 1960s, there are now hundreds of choices. Similarly, the number of magazines and radio stations have grown but each reaches fewer people, because they are highly targeted.
The Media Giants 
While there are plenty of independent media outlets, a relatively small number of large corporations control the biggest and most profitable media vehicles in the country. Some of the properties listed below are not fully owned by these conglomerates but they have a significant or controlling interest.
-Viacom owns a huge variety of properties, especially in television. Their holdings include MTV and Comedy Central, VH1, Paramount Pictures, Nickelodeon, Simon & Schuster, Scribner, Infinity Radio stations, CBS networks, TNN, CMT, BET, exclusive transit advertising in major markets, and billboards across the country.
-General Electric is a far reaching company that does everything from jet engines and futuristic plastics to television shows. TGE owns all the NBC networks, A&E and History channels, IFC, Bravo, AMC, and WE networks. They also own several sports teams and venues like the New York Knicks and Rangers, and Madison Square Garden.
-Disney not only has a themepark empire and their movie studios, they also own Miramax, ESPN, ABC networks, Hyperion Books, Buena Vista and Touchstone Pictures, Soap Net, Radio Disney, and they own the Mighty Ducks and Anaheim Angels sports teams, Broadway productions like The Lion King, and magazines like Family Fun. They also own a town called Celebration, Florida.
-Time Warner has a multitude of divisions. Their properties include AOL and Netscape, moviefone and mapquest, and Winamp, DC Comics, Warner Brothers, New Line Cinema, Castle Rock Productions, CNN networks, E! and Style Networks, Cartoon Network, HBO and Cinemax, Turner Classic Movies and Court TV, Atlantic, Elektra, and Rhino recording labels, and magazines like Time, Golf, Popular Science, Real Simple, and Entertainment Weekly. They are also a major cable provider.
-Clear Channel owns many radio stations and billboards. In some markets they nearly have a monopoly on the local radio market.
-News Corporation is owned by Australian Rupert Murdoch. His North American properties include Fox networks, FX, National Geographic Channel, TV Guide, Twentieth Century Fox, the New York Post, several British tabloids. These media outlets, especially FOX News, have been accused of conservative bias.
-Vivendi Universal owns Houghton Mifflin textbook publisher, Universal Studios and themepark, USA, Sci-Fi, Canal + networks, Interscope, Geffen, Def Jam, Mercury, and Motown Records. They also own multiple properties in France.
-Sony not only makes electronics equipment, they own Columbia Pictures and Revolution Studios, Loews Movie Theaters, and own the Columbia, American Epic, and Sony recording labels.
-Liberty Media owns the Discovery Channel, QVC, Primedia magazine group (Seventeen), Hallmark Network, and hotel internet access.
The following corporations also have strong properties, although they are not quite as large as the previously mentioned companies.
-Martha Stewart Omnimedia owns all media properties that carry the Martha Stewart name, including her television shows and magazines like Martha Stewart Living and Everyday Food.
-Conde Nast owns many high profile magazines like Wired, Vogue, Gourmet, the New Yorker. They also own other properties like the City Business Journals.
-Hearst owns several newspapers and magazines like Cosmopolitan and Oprah's magazine.
-Wenner Media owns magazines like Men's Journal, Maxim, Stuff, The Week, and Rolling Stone.