Because I haven't done one of these for a while...
In A Nutshell:- Young girl discovers she's the chosen one and gets to kill undead things and avert the apocalypse on an annual basis.
The Basics:- Having initially brought the story of Buffy Summers, the schoolgirl who is destined to be the vampire slayer, to the big screen with a distinct lack of success, Joss Whedon tried again on the small screen. And it was infinitely better. Over seven seasons, we follow Buffy and her friends as they grow up and save the world from unspeakable things which want to destroy it / rule it / both.
Why's It So Great Then?:- Well, I have to admit, I was resistant to watching it at first. It just looked like some twee and bog standard teen drama with a bit of a horror twinge but, once I actually sat down to watch it, I was completely hooked from the start. The key thing for me is that Joss Whedon knows how to throw in snappy, sparky dialogue and combines it with a strong knowledge of the genre and its associated cliches which meant he was able to sidestep them and blindside you from time to time just when you thought you knew where it was going. It's also a series that's unafraid to develop it's main characters and even dispose of them if the story calls for it. True, that's fairly common in TV drama these days but it didn't happen so much ten or so years ago.* It's also fairly unusual in that it spawned a spin-off series of equally high quality (Angel) and one that felt as if it had been cancelled before its time.
Surely Some Of It Must Be Rubbish:- Well, it's a TV drama with 22 episodes a year to churn out so, yeah, from time to time, they're not always brilliant. But when it's good, it's one of the best shows there is. It does, however, have a strangely puritanical "drugs are bad" attitude towards drinking. Or is it just that I'm a disgraceful, booze-sodden sot? That was rhetorical, you know, no need to leap straight in with answers. You people...
So We Should Seek Out This Thing Of Which You Speak?:- If you're put off by the fact that it's got people in bumpy headed make-up in it but are fond of a drama series which grow and change their characters as well as offering you story arcs to get drawn into, then definitely give it a go. It's good fun, it's got an ending and Sarah Michelle looks good in the earlier ones before she gets all skinny and loses her boobs. Oh, and Charisma Carpenter'd definitely get it, too.
* On a side note, I've started re-watching Star Trek: The Next Generation recently and it's odd to see how much TV has changed from the fairly formulaic stand-alone model of 22 years ago (yes, it is that old. Yikes.)