Giant

In general a guy who is somewhat larger than average by means besides obesity is stronger than a normal sized human and more dangerous in a scrap, either unarmed or armed with melee weapons. On the same note larger animals are more dangerous individually than smaller animals. Really tall humans that are more than 2 meters tall usually have major health concerns, which get more and more pronounced as height increases. Individuals more than 2.3 meters tall often have to walk with canes. Never the less, these fine details of medicine were not well known in the Bronze Age and people loved to embellish and embellish on embellishments in storytelling. As such it is little surprise that Giants, people who are of a much greater scale than normal Humans, are so common in mythology.

Mythological Giants

Aside from the cyclopes, who're the most famous but also have their own page, Greek mythology featured the Gigantes, a race of giants created by Gaia to try to overthrow the gods, just as the gods had once overthrown the titans. They did pretty well at it for a while, not least because Hera had foreseen that no god could destroy them, but Herakles eventually rolled up and helped finish them all off.

Norse giants, meanwhile, are the embodiments of powerful elemental forces. From the frost giants of Niflheim, to the fire giants of Muspelheim, the giants fought, slept with, and occasionally married the Norse pantheon as part of the endless struggle of civilization against nature. Notably, Loki, the trickster, was a giant sworn as Odin's brother and ally. And, of course, all creation ultimately ends in a final mighty battle between giants and gods, Ragnarok, in which the fire giant king Surtr ultimately emerges triumphant and burns all of Yggdrasil to a cinder.

Finally, there are the giants of Medieval Europe. Some were stupid, savage brutes, threats to knightly adventurers because of their incredible strength, while others were sophisticated beings that lived in the clouds, though no less evil. Both liked their human flesh, though the smarter ones cooked it first.

Giants in D&D

In Dungeons and Dragons "giant" can designate a subtype of monsters going from trolls, ogres, cyclopes and various Oni all the way up to the "true" giants. They draw from all of the mythologies above, by designating various types of (Whatever) Giant to split them based on habitat. Certain types of giants live in certain areas or have special abilties. For example, the Forest, Frost, Hill, Jungle, Mountain and Ocean Giants all live from where you expect them to. In one cases this is less clear (Fog, Stone and Sun Giants) or almost impossible to tell (Death and Eldritch Giants). Between the books there are often dozens of different giants available, with 3.5e having a staggering 22 different types of "true" giants and a host of others with the creature type.

The six "classic" flavors of giant (the most common ones, the way there're five "classic" colors of chromatic and metallic dragon each) are, arranged according to size:

• Hill giants; savage, stupid monsters who attack other races to sate their immense appetites. They look like big, fat, filthy humanoids.
• Fire giants; militaristic, heavily-armored soldiers who act kind of like big, evil dwarves, with their mastery of smithwork and penchant for slavery. They tend to have dark skin on a coffee-to-ashen-purple spectrum and (naturally) bright red hair. Completing the dwarf comparison, expect lots of beards, braids, and long, wild hair.
• Stone giants; master artisans who live underground quietly keeping to themselves, and are much faster than they look. They look like they're made out of living stone, and their tough skin gives them natural armor on top of their speed, and many can also catch projectiles out of the air.
• Frost giants; Norse-inspired raiders who live in the frozen north, taming huge monsters to accompany them on their quest for plunder. They look like blue-skinned vikings and dress like it too. The most famous frost giant in the Forgotten Realms is the adventurer known as Harshnag the Grim, who recently enjoyed an extended appearance in Storm King's Thunder.
• Cloud giants; who live in sky-castles, have magical powers, and tend to be witty sophisticates fond of games and art. Notable for their ideological split between those who're neutral good-aligned and those who're neutral evil-aligned. The least visually well-defined of the giants here, while they tend to have pale skin and blue hair, their skin might be blue, might be lavender, might just be white, and they might or might not have fangs.
• Storm giants; benevolent but distant sages who are experts at seeing into the future, and have not only even moar magical powers but the ability to chuck bolts of fucking lightning at things that annoy them. They can also breathe underwater. Dark blue, purple, or brown-skinned with white hair.

Giants tend to be in the mid to high range of monsters, starting as early as CR 7 for the Hill Giant all the way up to 26 for the Mountain Giant, and that's pre-class levels. As a general rule, the larger a giant is, the more powerful it is. The exceptions are the fire giants, who are barely smaller than stone giants but right below cloud giants on the heirarchy. Giants are very much a mixed bag, being anywhere on the range from bullies to militant assholes to dicks sitting in the clouds and doing nothing. This can make them decent enemies, aloof sages, or a nice way to make players feel like they're hot shit before delivering the smackdown. Like dragons, each of their subraces tends to have a "racial alignment" to them.

They are often of the Large or Huge size categories and are all experts at the art of throwing boulders, which do a shitload of damage thanks to their immense size. Hilariously, this is true even for the otherwise well-armed giants who're masters of fabricating quality weapons, like fire giants. Most settings have them as remnants of an ancient giant empire that has since slipped into sad decline, and they tend to be bitter rivals of dragons. Notably, even when they have ideological/alignment-based disagreements, giants tend to get along among themselves, with bigger giants looking after smaller ones and smaller ones in turn mostly doing what bigger ones say.

A 5e module, Storm King's Thunder, is heavily giant-centric, and has put them into the spotlight after years in the margins. And, yes, there are examples of sexy lady giants in all the flavors above. Except maybe Hill. Unless you're into that whole fat-stupid-and-unwashed thing. We're not here to judge.

Giants in modern fantasy

In A Song of Ice and Fire, giants are huge hairy dim witted hominids about three to four meters tall that live north of the wall. Even so thats peanuts by the standards of most giants.