If I were to refer to a giant, you'd have a pretty good idea what I mean, right? Or would you? While giants are all big and humanoid, they vary in a lot of ways. Some are only slightly larger than normal humans, while others are truly huge. Goliath's height is given in different versions of the Bible as "four cubits and a span" (about six and a half feet) and "six cubits and a span" (about nine and a half feet). The Nephilim are said in the Book of Enoch to be 300 cubits (about 450 feet) in height. The Gigantes and Hecatonchires were presumably large enough to move mountains. Most of the Norse Jotun didn't seem to be much bigger than humans or Aesir, but their ruler Utgard-Loki had a glove that Thor and his companions mistook for a building, and the primordial giant Ymir must have been planet-sized. When a creature can vary from eight feet tall to positively Himalayan, they're probably not all the same species, are they? Then again, using scientific terminology for beings that couldn't really exist, as the Square-Cube Law means a human frame that big would collapse instantly, might not be the best idea.
So what about ogres? As far as fairy tales go, I don't know that there's a whole lot of difference between giants and ogres. Well, that's not entirely true. It's more that there doesn't HAVE to be a difference. A large being that's basically humanoid but with grotesque features that eats regular-sized humans could be called either a giant or an ogre. The thing is, however, that some giants of mythology and folklore are friendly, helpful, intelligent, and even beautiful. Ogres pretty much HAVE to be mean, ugly, and dim-witted, or they wouldn't be ogres. I guess they could be considered a subset of giants that are less human than their fellows. The Wikipedia page says that the term dates back to twelfth-century French, and gives several possible derivations.
There is, perhaps, even more confusion over trolls. These creatures of Norse mythology are often more or less interchangeable with giants and ogres, but other traditions say that they are essentially human-sized dwellers of forests and underground caves. Trolls are generally considered to have magical powers, and in some parts of Scandinavia, the stories told about them were similar to ones about fairies in other parts of Europe. Modern popular culture still provides several different takes on trolls. The Scandinavian folk tale of the Three Billy-Goats Gruff makes its troll a ravenous creature that lives under a bridge and is easily fooled, which would make it not all that different from your typical ogre. But we also can't forget the troll dolls that gained popularity in the early sixties and have enjoyed occasional resurgences since then. I remember them being big in the mid-nineties, when I was in high school. These trolls are hardly ogrish brutes, but instead cute creatures with brightly-colored hair. Tolkien's trolls are large and uncouth humanoids that turn to stone in the daylight, a trait that he probably took from tales of the Norse dwarves. Terry Pratchett's Discworld series expands upon this idea of Tolkien's by having trolls made of rock, and saying that they freeze up in the daylight because they can't handle the heat. Their silicon-based brains are similar in operation to computers, so trolls in warmer areas tend not to be very bright. Sunscreen and devices like the cooling helmet Sergeant Detritus wears have enabled trolls to function more smoothly in cities. Oh, and for what it's worth, the term "troll" for an Internet agitator presumably comes not from the monsters but from the fishing term, although it works pretty well with both definitions.