Dimensions & Alternate Realities
Buffy's dimension is not the only one. As Anya points out, there are thousands of others that co-exist and several are mentioned on the show, some are even shown. These dimensions can be made accessible by portals, others created by spells.
The first time we an altered reality is in season one episode 'Nightmares', when everyone's worse nightmares come true. Giles can no longer read, and Buffy becomes a vampire. In this case everyone is fully conscious that something is not right. They realise that reality as they know it has been changed. This was created by a young body in a coma, trapped in his own nightmare.
Hell Dimension I - AcathlaWhen Angelus sets the apocalyptic demon Acathla free, opening a portal that will suck Buffy's world into a Hell dimension, the only way for Buffy to save the world is to shed Angelus' blood. She sends him to Hell as he gets his soul back. Time is warped and the few months he stayed trapped there compare to a hundred human years. This Hell dimension is apparently full of demons that torture anything good (i.e. with a soul).
Hell Dimension II - I'm nobody
When hiding in L.A., Buffy finds herself taken in a Hell dimension. Time there is also warped, one human day is the equivalent of almost a hundred years. To enter this dimension, you must go through a black pool. Buffy discovers a world where humans are reduced to slavery and loose their identity. Once they are too old to work, the demons bring them back to our dimension to die.
What is Buffy never came to Sunnydale?
When Coredlia wishes that Buffy have never set foot in Sunnydale, Anyanka creates an alternate universe in which Buffy lives in Cleveland and the town is ruled by a group of vampires led by the master, free from his underground prison. Life in Sunnydale is very different and the kids are far less carefree, since many of them are dead! The streets are empty at night and the Bronze has been taken over by the living dead. Xander and Willow are vampires. Giles, Oz and Larry try to fight back, but it doesn't change much. When Buffy finally arrives, she fights the Master, who kills her, fulfilling the Prophecy. Willow and Angel are dusted. Giles manages to terminate this universe by destroying Anyanka's powercentre, bringing everything back to normal, as if nothing had happened.
This is D'Hoffryn's and the vengeance demons dimension. It basically seems to be just a dark place! We see it in Doppleganglander, when Anya demands her powers back, in Something Blue, when D'Hoffryn want to turn Willow into a vengeance demon and in Hells Bells when Anya gets her powers back. D'Hoffryn travels from one dimension to another using a portal that leaves a burnt circle on the ground.
Orange demon land
When Buffy goes to college, she winds up sharing a room with Kathy, who turns out to be a demon escapee from another dimension. Her father comes looking for her and takes her home through a portal that opens up in the ground and swallows them.
The House of Fear
Preparing Halloween, a few frat boys unknowingly summon Gachnar, the demon of fear, causing their party to turn into Hell when all the decorations become real and everyone's most secret fears are exposed. Gachnar feeds on all the fear to get stronger and rise, but he's only a few inches tall...
When pre-troika - but nonetheless dorky - Jonathan casts a spell to be loved and adulated by everyone, he creates an altered reality where he is a super hero -he took down the Master and the Mayor, an acclaimed singer - his album has just come out, an actor - he starred in 'The Matrix', a writer, a sportsman, a military consultant... basically, he does everything and he does it well. He's even got his own swimsuit calendar *shudder*. Women want him and men want to be him.
To protect the Key, several monks made it human and sent it to the Slayer. They also created an altered reality in which Buffy has a sister, Dawn - who is in fact the Key. Fourteen years of fake memories were created in everybody's mind and they are fully convinced that Dawn is real. Buffy is the first to see through it and tells Giles. Joyce also figures something is wrong when she is delirious in hospital. Buffy finally tells the rest of the gang and Dawn eventually finds out too. Crazy people and certain animals can easily see through it.
The Troll world
When Olaf, Anya's ex-boyfriend turned troll is set free in Sunnydale, Willow and Anya cast a spell to send him back to the troll world, at least they hope, since, as Anya points out, dimensions are very unstable.
Hell Dimension III - Glory
Glory comes from yet another Hell dimension that she rules with two other gods, until they decided to kick her out (I wonder why...) landing her in Buffy's dimension, desperately searching the key to open all dimensional portals and return home.
When Buffy died, she was sent to another dimension, a paradise dimension, where she was finally at peace. The rest of the gang, convinced she was trapped in Hell dimension, cast a spell to bring her back to life.
Xander summons a musical demon to cheer everyone up and the whole town starts breaking into a song, telling everyone their most hidden secrets. The demon apparently comes from a Hell dimension and absolutely wants to take Dawn back with him to marry her.
Is Buffy crazy?
After a ruff encounter with a demon that injects some liquid into her arm, Buffy starts to hallucinate. She sees a very different world; where's she's still in L.A., shut up in a mental institution. The doctors say she is delirious and has created herself a world with family and friends where she is some kind of super hero.