Giants are some of the most fearsome enemies encountered in Skyrim. These 15 foot tall monsters canusually be found wandering the tundra east of Whiterun and to the west near the hot springs area (Their camps are marked with mammoth skull symbols on the map). They can be encountered relatively early, compared to other high caliber enemies like dragons. They are capable of easily one-hitting low level players. This can usually lead to a visually comic glitch where anyone killed by a swing of a giant's club will go flying hundreds of feet into the air. They are also usually seen alongside mammoths, which will all turn hostile if any of them (giants or mammoths) are attacked.
Strategy for fighting giants at low levels usually involves using ranged weapons/spells and exploiting the AI's inability to jump or enter doorways. For example, if you have a bow and an ample supply of arrows (ordestruction spells with good range), you can jump on a rock that isn't accessible without jumping, the giants and mammoths will usually just sit there and stare at you while you hit them in the face with arrows and lightning bolts.
Farming of giants can be very lucrative since they usually drop large amounts of gold (this seems to be level specific, while killing them at level 2 using the jumping on a rock and shooting arrows exploit, they usually dropped gold in the 100-150 range, while at level 25 they would drop around 300.) They also drop a Giant's Toe, an Alchemy ingredient that can be used to make high-level potions such as Fortify Health.
Giants are also surprisingly vulnerable to a fully unlocked Unrelenting Force shout but the mammoths are not. This is another good strategy since blowing a Giant off a 20 foot cliff with Unrelenting Force is an instant kill.