A super-speed villain, in the DC Universe. First appeared in Flash #139.
Professor Zoom (real name Eobard Thawne) is also known as the Reverse-Flash, mainly because his speed was equal to the Flash's, and his costume is identical to the (Silver Age) Flash's; a yellow jumpsuit with red lightning around the forearms and waist, red boots, small red wings just above the ears, and on the chest a black circle with a red lightning bolt through it.
Professor Zoom (he's not really a professor, he just calls himself that) hailed from the 25th century, and was at first your average villain, doing bank robberies and being foiled by the Flash, but he soon graduated to major league villainy, and became one of Barry Allen's deadliest foes. He set out to cause Barry Allen as much pain as possible, and killed Iris, Barry's wife, by vibrating his hand through her skull (only she didn't really die - she was recalled to the 30th century where she was born ... it's kinda complicated).
Professor Zoom hung around for a while, indulging in the more villainous stuff, and was a member of the Secret Society of Super-Villains at one time, but when he heard that Barry Allen was getting married again he couldn't resist causing Barry more pain. He taunted Barry about how he was going to kill his new wife on their wedding day, and Barry obviously was going to do anything to stop him. They fought, and Barry chased Zoom all over the place, but eventually, with only microseconds to go before Zoom plunged his hand into Barry's fiancée's skull, Barry stopped him the only way he could, by breaking his neck.
Zoom died, but that was effectively the end of Barry's career, as he was arrested and tried for murder. Although he was eventually acquitted (after a long, long trial) he left the 20th century to join Iris in the 30th, but soon after died saving the universe.
You might think that that would be the last we'd hear of Professor Zoom, him being dead and all, but you're forgetting that he's from the 25th century. Through the magic of time travel he can turn up anytime. And indeed, he did. It was when he turned up after Barry was dead and met Wally West that we learned more of his origins.
Before he became the Reverse-Flash, Eobard Thawne, a resident of Central City in the 25th century, was a big fan of super-heroes. Even though there weren't any around in the 25th century (they'd make a big comeback in the 30th, though) he was fascinated by them, and was a particularly big fan of Barry Allen, the Flash. He had all the important Flash collectibles, and even had hid face altered so he looked just like Barry Allen. One day he found the ultimate Flash collectible - the Cosmic Treadmill, and came up with the idea of going back in time to meet his hero. Of course, the Cosmic Treadmill doesn't work unless you can run really, really fast, so he spent a fortune using 25th century technology to give himself super-speed. Thus prepared, he hopped on the treadmill and zoomed into the past.
Disoriented by the journey, Thawne found his way to the Flash Museum, where he was dismayed to discover that, unfortunately, he'd obviously not read the manual, and ended up not in Barry Allen's time, but a few years later, after Barry had died saving the universe, and been replaced as Flash by his nephew, Wally West. Looking around the museum a bit more, Thawne was also shocked to discover the identity of Barry Allen's worst enemy. Professor Zoom, the Reverse-Flash, was none other than Eobard Thawne! Holy temporal paradox!
Thoroughly disoriented by this information, Thawne had a bit of a mental breakdown, and came out of it convinced that he had been sent back to this time not to meet Barry Allen, but to replace him. He grabbed Barry's old costume ring from a nearby display, suited up, and headed over to Jay Garrick's house to spread the good news that Barry Allen was back from the dead.
Of course, as he was a little unstable, it wasn't too long before Wally and all the other speedsters realised something was up with their old friend. The real giveaway was when "Barry" left Wally to die in a death trap they'd been caught in. Thawne was exposed, and Wally gave him a hiding and sent him back where he came from, where he arrived disoriented (again!), holding a fragment of the Reverse-Flash costume, and forgetting everything except that he really, really hated the Flash.
He popped up once or twice more, and we later learned that he was descended from Barry Allen's evil twin brother, also known as Cobalt Blue.