Through the acquisition, CNET will gain Ziff's technology publication Computer Shopper, online educational service Smart Planet and online technology information site ZDNet.
CNET chief executive Shelby Bonnie will remain CEO, while Ziff CEO Dan Rosensweig will become president. CNET president Richard Marino will become chief operating officer.
"Technology is driving the global economy," Bonnie said. "Our union accelerates our ability to participate in this multitrillion-dollar global market throughout the supply chain."
Added Rosensweig: "We're creating a company with global reach and anticipated revenues in excess of half a billion dollars next year."
CNET said it will exchange 0.59 shares of CNET stock for each ZDNet share and 0.34 shares for each Ziff share. Based on CNET's closing price yesterday of $32.18, the transaction is valued around $1.6 billion. CNET said it will issue a total of 50 million shares.
As a result, Ziff stockholders will hold about 35 percent of the combined equity. The deal is subject to approval by shareholders. Both sides expect the deal to close in the fourth quarter.
Combined, the companies have an online audience of 16.6 million unique monthly users and would rank as the eighth largest property on the Internet with a reach of 22 percent, according to a statement. The companies will have cash and marketable securities of about $600 million.
Ziff investor Softbank is backing the deal.
"We enthusiastically support the union of these two industry leaders," said Softbank president Masayoshi Son. A Softbank representative will join CNET's board.
San Francisco-based CNET runs an Internet network with news on technology and information on computer products, and it provides programming for radio and television that is broadcast in almost 100 countries around the world. CNET is the publisher of News.com
Last year CNET bought 10 companies. In February, it bought MySimon, a comparison-shopping service, and last week it announced that it acquired closely held Apollo Solutions.
Ziff recently spun off its events business, which included Comdex, one of the world's largest technology fairs; NetWorld+Interop; and Seybold Seminars.
Lazard Freres was CNET's financial adviser, and Morgan Stanley served as financial adviser to ZDNet.