The Giants were the main antagonists of the novel and film of The BFG and were led by the Fleshlumpeater, being the most notable member of the band of monsters. Compared to the BFG, they were ferocious, hideous monsters.
In the novel
Living in a large desert known as Giant Country, (implied to be on another planet or in another dimension) the Giants were fifty foot tall, and often engaged in night-time raids of the human world, stealing men, women and children alike under the cover of night and devouring them: although the BFG was aware of the crimes his fellow Giants committed he was unable to stop their evil ways.
This would change when the BFG befriended a young girl named Sophie and took her to Giant Land, through her, he picked up the courage to finally call on the British Army to deal with the Giants and they managed to capture the beasts as they slept but woke up the Fleshlumpeater when doing so. The Fleshlumpeater was surprised, but he was even more hungry at seeing the humans in his country, and he jumped to his feet, boasting about how he would eat them all. Sophie stuck a brooch the Queen had given her into the Fleshlumpeater, causing him pain and making him retreat, the BFG tricked him into being tied up. Afterwards, they flew the Giants back to England by chaining them under helicopters and had them locked away in a large pit where they would be fed nothing but a disgusting plant known as "Snozzcumbers" as punishment for their crimes for the rest of their lives.
The BFG (2016 film)
In the upcoming film of The BFG, the Giants are once again the main antagonists. The Fleshlumpeater will be portrayed by Jemaine Clement, who also portrayed Boris the Animal, and Bill Hader will portray the Bloodbottler.
(Note: This only talks about the evil Giants, the BFG is naturally not included) The Giants see humans only as food. They hate humans and only love them for their luscious taste, apparently, all humans taste different, and thus reflect the nature of their countries - for instance, humans from Turkey taste strongly of turkey, the animal. Welsh humans have a very suspicious taste, apparently, tasting strongly of whales, and mostly all Giants avoid humans from Greece, because apparently they taste greasy and no Giants eat Greeks ever (a possible reference to how humanity's greatest Giant slayers have been from ancient Greek stories). All the Giants are dim-witted, with the exception of two, the Fleshlumpeater and the Bloodbottler, and the Fleshlumpeater has been their sole leader since perhaps the dawn of time. All the Giants are incredibly violent and love hurting each other in rough brawls (similar to schoolyard fights) however according to the BFG they will never go so far as to kill or eat one of their own species - but even the Fleshlumpeater breaks this code of honour at the end by vowing to eat the BFG for a change. The Fleshlumpeater is the most sadistic and apparently likes spending his days planning which humans to eat in the night time.
However, routine can be deadly, and the BFG and Sophie are able to exploit the routine of the dim-witted Giants by using their absence when hunting humans to approach the Queen to send the Army to imprison them all.
The Giants are all terrified of Jack because he is one of the only humans to have killed a Giant and thus they know he is more than a match for them. Luckily for them, Jack may or may not be fiction.