The Buffy Omnibus
collects the old Dark Horse Comics series that preceded Buffy: Season Eight
. These stories were not written by Joss Whedon or with Whedon's input, which shows as there isn't the characteristic clever dialog or planting of plot seeds that emerge later.
Omnibus Volume 1 tells Buffy's "origin" story, which means the first part is a graphic adaptation of the original movie, and the rest is the events between the end of the movie (when Buffy burned down her old high school) and the beginning of Season One of the TV show, when she arrived in Sunnydale.
Many of the events were referred to in the TV series, so it can be considered "canon" in the same way the Season Eight comics series is. Here is the story of when Buffy ran away to Vegas with her boyfriend, Pike, returned home to parents fighting and on the verge of divorce, and then spent some time being institutionalized after they (i.e., Dawn) found her diary. Naturally, Buffy being the Slayer, everywhere she goes turns out to be a den of vampires or demons, and she can't leave before bringing down the place in flames.
The inclusion of Dawn is problematic, given that she didn't even appear until Season Four of the TV show. But the nature of her origin means she "existed" retroactively, so the writers decided to include her in the storyline. And she gets a story of her own about a misdelivered teddy bear (which, of course, turn out to be another demonic plot).
It's a pretty good collection, assuming you are a Buffy fan. If you're not, then you'll probably find it nothing special. The artwork is good though not exceptional. I do wish they'd get all the artists on the same page about whether Buffy's eyes are green or blue, though.