Cain and Abel

Publication History
At first, Cain and Abel were EC Comic-esqe horror comic hosts in House of Mystery and House of Secrets, which ran from the 1950s to the early 1980s. During the 1970's, they co-hosted a humor / horror anthology comic called Plop!. On a letter page in Weird Mystery Tales # 3, Destiny claimed Cain, Abel and Eve were not their Biblical Counterparts. The duo and Eve hosted Secrets of Haunted House, until it became Destiny's title. House of Mystery was canceled. Cain appeared in the mostly-reprint Welcome Back to the House of Mystery series. The House of Secret series (along with The Witching Hour) were merged with The Unexpected, due to the infamous DC Implosion. Neil Gaiman revamped them for his new Sandman-series.
Fictional History
House of Mystery / House of Secrets
Cain and Able were care-takers of the House of Mystery (Cain) and the House of Secrets (Abel). Cain usually told tales to various people that stayed at the House of Mystery, while Abel (in the early issues) told his to his "imaginary" ("imaginary" was always written in quotes) friend Goldie, but he always appeared to be alone. Abel claimed Goldie was a ghost. Notably, Abel was very nice (even to his brother) and it's unclear why Cain was abusive. Both of the houses they worked at / lived in were on opposite sides a graveyard. Cain had a dragon-like gargoyle, who ate that reality's version of real-life writer Paul Kupperberg and editor Jack C. Harris. The houses were in a small town in Kentucky. Abel had to temporally move in with Cain, as the House of Secrets was being transported at the time. The final issue of House of Mystery, the titular house was torn down.
Pre-Sandman
In 1985, Alan Moore revamped them so they could retell the Swamp Thing's origin. In Blue Devil #20, Cain is seen taking care of the "House of Weirdness", until the titular character moves in. During the Crisis, after finding the House of Mystery, Elvira tried to find the-then-missing Cain. It's revealed he was still at the House of Weirdness with Blue Devil. In Swamp Thing #49-50, they watched a fight in Hell and acted like commentators. They made some less notable appearances, in which Cain tried to restrain himself from killing Abel and failed.
Sandman
It's implied, in a conservation between Cain and Lucifer (that was post-crisis), that Cain and Abel are their Biblical counterparts. It should be noted pre-Crisis Destiny claimed they were not. It's became even more unclear if they are their Biblical counterparts in this series. In this verison, Cain and Abel are inhabants of the Dreaming. This verison of Cain is much more aggressive towards Abel than past verisons, but Abel usually came back to life (due to being a dream). Cain hurts Abel, due to a " macabre form of obsessive-compulsive disorder". Despite abusing him, Cain seems to care about him. As such in The Sandman: The Wake, he became upset when Abel permanently died, to the point he conviced the new Lord of Dreams to bring him back. In Sandman # 2, Cain gave Abel a gargoyle, but got mad when Abel didn't want to give it a name that started with "G". So, Abel decided to call the gargoyle "Goldie". Despite usually being comic relief, the duo did play some imporant roles in the series.
Later...
Both Cain and Abel were main characters in the new House of Mystery 2008 Vertigo series.
References:
http://en.wikipedia.org/wiki/Cain_and_Abel_(comics)
http://upload.wikimedia.org/wikipedia/en/a/a4/Cain_%26_Able.jpg