WoW Forever realmless explained: realm list, realm types, transfers and population

Yes, WoW Forever is realmless. There is no realm list: you choose a ruleset when you make a character, and each ruleset works as one large community for your region. So searches for a "realm list" have no real answer, and you don't need to pick a server. All of this is Confirmed from Blizzard's ruleset post and return guide unless marked.

Realm types (rulesets)

Normal: PvP only with mutual consent.
PvP: you're almost always at risk from the other faction.
Roleplay: strict naming and behaviour rules.
Roleplay PvP: Roleplay rules plus PvP.
Hardcore: arrives some time after launch. See Hardcore.

More help choosing is in Normal, PvP or Roleplay.

Who you can play with

Groups, guilds, dungeons and raids only work within one ruleset and faction.
Battlegrounds can mix players from different rulesets, except Hardcore, which stays separate.
Names are unique across your whole region, not per server.

Realm transfers

Blizzard says players can't freely hop between rulesets. To play on another one, you make a new character there. The one exception named is Hardcore: a Hardcore character that dies can transfer to another ruleset, including PvP. A paid transfer service isn't mentioned in the posts we've read, so we don't know whether one will appear.

Population

Unknown. Blizzard hasn't published population numbers for any ruleset, and we haven't seen a reliable tracker for a game that isn't out yet. Treat any "realm population" site with caution. Servers can't be "full" in the old sense if there's no list of them, but Blizzard hasn't explained launch capacity or queues. See can't log in or servers down.

PvP faction balance

On the PvP ruleset, you can only make characters for one faction. Blizzard's example: after an Alliance character there, you can't make a Horde one.
Forever has systems to keep the factions closer in number, like Season of Discovery. Character creation for one faction can be limited for a while.
Blizzard hasn't published the current Horde and Alliance split. Players on the forums report one, but we can't check their numbers.

See can't create a Horde character if you've hit that lock.

Layers

Unknown. Blizzard hasn't documented how the world is split up within a ruleset. See our quick answers.