Raptor versus Megaraptor
What's a "raptor"?
Today "raptor" refers to voracious, bipedal dinosaurs with "killing claws," toe claws that never touch the ground and thus are always sharp. Velociraptor is the most famous thanks to Jurassic Park, though, in the movie, these raptors were up to 15' long and nearly 200 lbs, while in real life Velociraptor was a tiny thing, no more than 30 lbs and, with a tail, 6' long. If you are picturing a turkey with a tail, or a medium-sized dog, you are thinking of the right-sized animal. However, even though it was small, it had a 2"+ "killing claw" that would have easily eviscerated most prey.
Figure 1. Velociraptor
Utahraptor is the largest ever discovered at over 20' long and roughly 1,000 lbs. It was named in 1993 and made a big splash, proving that "raptors" were even larger than those in Jurassic Park! The "killing claw" is over 6" long, probably over 9" when one factors in the keratinous sheath that covered the claw bone! Pound-for-pound one of the most dangerous animals that ever lived, Utahraptor had the speed of a big cat, claws that shame a grizzly bear, and the power of a crocodile. And we haven't mentioned its mouthful of teeth and hands armed with sharp, highly recurved claws!
Figure 2. Utahraptor
Paleontologists call these kinds of "raptors" dromaeosaurids, named after the first one discovered, Dromaeosaurus. However, dinosaurs ending in "raptor" aren't all related! Raptor means "thief" or "robber" in Latin and has been used by paleontologists for many different dinosaurs. Here is a brief list of dinosaurs that end in "raptor" that aren't dromaeosaurids at all! Oviraptor "egg thief," named because it was originally thought to be robbing eggs, turned out to be nesting on its own eggs and thus was misnamed. Sinraptor, "Chinese thief," is actually a metriocanthorsaurid, a kind of large, "primitive" theropod. Conchoraptor "mussel thief" when named was believed to eat mollusks. Eoraptor, "dawn thief", was one of the earliest known true dinosaurs.
So what's a "megaraptorid"?
A series of misidentifications, mostly because paleontologists were unknowingly trying to figure out a previously unknown group of meat-eating dinosaurs, led to the use of "raptor" on dinosaurs that aren't remotely related to Velociraptor, Utahraptor, and their cousins.
In the late 1990s and throughout the 2000s, giant "killing claws" were found in South America, Asia, and Australia and attributed to huge "raptor" dinosaurs. Megaraptor was the first of these supergiant raptors. The 10" claw was interpreted as a huge "killing claw" belonging to an enormous "raptor," one dwarfing even Utahraptor! A large claw discovered in Japan (later to be named Fukuiraptor) was identified as a "killing claw" belonging to a giant "raptor." Additional dinosaurs with large claws were named in the 2000s: Orkoraptor, Aerosteon, and Australovenator.
Figure 4. New, unnamed megaraptorid restoration
It wasn't until 2010 that paleontologists realized the gigantic toe "killing claws" were actually hand claws, and these "giant raptors" belonged to a completely new kind of southern hemisphere predatory dinosaur! The rules on dinosaur naming meant this group of dinosaurs would be forever called Megaraptorids, named after the first one of the group that was named (Megaraptor), even though none of these dinosaurs are closely related to dromaeosaurids, the traditional "raptors" from Jurassic Park.
Today, no one is quite sure what these animals are related to. There is a huge debate in the paleontological community about what group they evolved from. Depending on the study, their closest relatives are allosaurids, coelurosaurs, neovenatorids, spinosaurids, or tyrannosaurids. With that many conclusions, the only way to resolve this is to find better material! If anyone needs me, I'll be in the field. :-)