Speculative fiction is an umbrella term that should include tales of fantasy and horror that are traditionaly excluded from strict science fiction. It also draws a separation line to exclude all the fiction that is entirely bereft of speculation.
These terms head up two broad schools of thought, which still trade ideological jabs half a century later. The best description of the schism I have ever read is quoted here in its entirety.
"Here is the difference:
Science Fiction is the serious realm of speculative literature that deals with such interesting speculations as aerospace travel, intelligent life on other planets, futuristic weaponry, and speculations into areas otherwise taboo, such as an enlightened approach to sexuality, that other genres shy away from.
Sci-Fi is the pulpish hack writing that deals with such geekish ideas as rocketships, bug-eyed aliens, rayguns, and orgies with hottie space-babes!"
As we can see, there is no relation whatsoever between the two. - John Wright, November 24, 2008 Comment on SF Signal.com
Hats off to you John Wright for setting us all straight.