Looking for quick summary of background story

Can someone point me towards a quick summary of the background story behind V:tES and info on the different clans/sects?

Well, most of it is tied to the Vampire: The Masquerade universe, the "Old World of Darkness".

The easiest summary is:
Caine was cursed by God to be a vampire.
Caine fathered some other vampires who all had different traits.
Noah's Flood happened (Vampires from before the Flood are Antediluvians)
The vampires who shared a vampiric ancestor shared traits and became clans.
The clans squabbled amongst themselves, got involved in human politics at times and at other times just stayed out of trouble.
The vampires split into two major factions: "Play nice with humans" (aka Camarilla) and "Humans are sheep/cows for Vampires" (aka Sabbat).
The common ancestor of all Ravnos woke up from a long torpid state and then devoured most of its descendents in a blood rampage called The Week of Nightmares until wizards literally blew the vampire up.
Someone tinkered with something and a bunch of previously dead vampires turned up from far beyond the grave.
Then the signs of the end times happened (aka Gehenna) where blood didn't seem to work, vampiric powers seemed to lose effect, a bright red star shone all through the night but could only be seen by supernatural beings.

Wikipedia etc probably has plenty of info on clans and sects.

Searching info from Vampire the Masquerade will yield you more info than searching about VTES.

Clans:

There was caine. Cain sired, by most accounts, three vampires:
Zillah, Irad, and Enoch

Those three sired the founding antideluvians:
Malkav(malkavians)
Brujah
Ennoia(gangrel)
Saulot(salubri)
Haqquim(assamite)
Set(umm... setites)
Cappadoccious(cappadoccians)

And then i either dont know names or there arent names for:
Ravnos, nosferatu, Lasombra, tzimitze, ventrue, tzimitze.
There are more antideluvians who arent founders.

Now, Cappadocciois was an oddball, and got duped around when he felt guilty and tried to downsize his clan. The harbingers of skulls are cappadoccians who were sealed in a catacomb then trapped in the underworld until the last great maelstorm in the underworld threw them out again, around 1999. Cappadoccious was betrayed by Augustus Giovanni, and his clan eaten amd usurped by the Giovanni.

Triole killed brujah, and true bru are his line, normal modern brujah hers.

Saulot got around alot. He made both warrior and healer saulubri, and accidentally made the Baali. He was obsessed with making a vampire with an avatarxable to preform true magik, and kinda duped Tremere into eating him, to take over Tremeres body eventually.

Other clans have offshoots and bloodlines, almost all antitribu are sabbat versions of clans.

The camarella exists to hide with humans, deny the existance of antideluviams, and sabbat to farm humans, lose track of ever being one, and kill antideluvians.

Anarchs are in modern times an offshoot of cam cities mostly, to avoid camarella nonsense.
Laibon are mostly bloodlines, osebo are brujah, akunanse gangrel, ishtari toreador. Guruhi are probably either no(odd, i know) or less likely, thexline of another antideluvian.
Hope i helped be simple. Theres alot to it.

Why?