Worlds of Tomorrow was a digest-sized SF magazine (162 pages) that was published from April, 1963 (Volume 1, Number 1), until Spring, 1971 ((Volume 5, Number 3), for a total run of 26 issues. The magazine appeared bi-monthly until Volume Four (1966), when it began to appear quarterly.
Frederik Pohl was editor from April, 1963, until May, 1967. At the time Pohl was also editing Galaxy and If. Ejler Jakobsson was editor during 1970-1971. It was not published during 1968-1969, but was combined with If. Galaxy Publishing Corp. was publisher during the magazine's first run. UPD Publishing Corp. was the publisher during the 1970-1971 run.
When it appeared in 1963, Worlds of Tomorrow was the first new SF magazine since 1958, and readers were hopeful that it would be a success. After its demise, critic Mike Ashley reported that the editorial policy for Worlds of Tomorrow fell somewhere between those of Galaxy and If. Editor Pohl had stated that the magazine "would not specialize in any one kind of science-fiction story," in the hope that such a policy would encourage writers to experiment.
Brian Stableford, writing in Peter Nicholls' The Science Fiction Encyclopedia, stated that notable stories to appear in Worlds of Tomorrow included Philip K. Dick's "All We Marsmen" (August - December, 1963); Samuel R. Delany's "The Star Pit" (February, 1967) [Delany's first story in any SF magazine]; and several stories in Philip Jose Farmer's "Riverworld' series. Another fan favorite was A. E. van Vogt's "The Ultra Man" (May, 1966). Stories from WoT received a number of Hugo nominations.
Starting with the November, 1965, issue, Worlds of Tomorrow took over publication of Sam Moskowitz's series of articles on themes and personalities in SF. This series had been running in Amazing until there was a change in publisher.
When the magazine was revived in 1970 under Jakobsson, writers included George H. Smith, Noel Loomis, Piers Anthony, Neal Barrett, Jr., and William Rotsler. Several of their stories reflected the new sexual freedom found in SF stories in the late 1960s.
For more on WoT, see http://www.sf-encyclopedia.com/entry/worlds_of_tomorrow