The five main elements in the Kingdom of Loathing are hot, cold, stench, spooky, and sleaze. Various creatures are aligned to different elements; if you prepare correctly, you can exploit their weaknesses and build up resistance to their attacks using various items and spells.
Some equipment adds bonus elemental damage to normal attacks. Sauceror spells deal either hot or cold damage, while Pastamancer spells can deal all five types in addition to purely physical damage. There are a few items and a skill which will auto-magically attack enemies with an element against which they are weak (where applicable). There are five special gravy fairy familiars, each aligned to one of the five elements.
There are other, more mysterious Non-Standard Elements which do not fit with the others. Only the monsters in The Valley of Rof L'm Fao deal bad spelling damage, only your Shadow deals unresistable shadow damage, and only monsters in the Future deal cute damage... and even then, no one does, but protect yourself just in case.
Learn more at Elements.