A Giant is a very common folkloric monster, as well as a regular antagonist in fairy tales, these beings are often confused with Ogres and vice versa and although the words can be interchanged their are a number of differences between a true giant and an ogre both in folklore and in fairy tales (although fairy tales tend to confuse the two monsters greatly).
Giants are often depicted as humans of abnormal size - often hundreds of feet tall - unlike the monstrous and often ugly Ogre, a Giant tended to be depicted as very much a normal human, albeit greatly magnified: in some cultures (most notably the Norse) Giants were also seen as embodiments of nature itself and were seen as formidable opponents of the gods (perhaps a throwback to memories of the Titans, a similar race of gigantic beings that warred with the Olympian gods in antiquity).
Although not as ugly or stupid as Ogres the Giant was seen as cruel and often cannibalistic, he would lord over some distant domain and often horded treasure or other goods and would sometimes attack villages or travellers in his spare time - also unlike the almost universally evil Ogre the Giant was seen as a being that could choose to be good or evil and a great many Giants aided humanity as well as opposed it.
Much like the Ogre a female Giant was known as a Giantess and she was often depicted as strikingly beautiful - in most fairy tales and folklore the Giantess was almost universally peaceful and when malignant was more of a deciever or trickster than a brute, though exceptions existed: such as a pair of Giantess sisters who tried to crush the Norse god Thor via lifting a chair he was sitting on up onto a roof only for Thor to snap their backs with his superior strength.
Although formidable opponents a number of legendary heroes made themselves famous by becoming Giant-Killers and even in modern usage the word "Giant-Killer" is used to describe someone to overcomes the odds: two of the most famous Giant-Killers of lore are Jack and Thor.