Instanced PvP battlegrounds are the only true Cross-Realmed content is in Classic Era. All realms within a geographic region (North America, for example) queue up for the same BGs no matter the realm type. PvE realms will queue with PvP/RP realms and Eastcoast will queue with Westcoast. This has been the case ever since the release of phase 2 content.
Realm Connections is what’s being referred to when players mention different clusters. These were implemented at the start of tbc prepatch and function as permanent links between realms of a given ruleset/timzeone. The realms inside a given cluster are effectively merged and players within the same cluster can fully trade/group/guild/raid/etc. with one another. Below is a piece of the bluepost detailing which realms are in which cluster:
Common names for the above clusters are given based on their ruleset/timezone:
PvP West
Oceanic
PvE East
PvE West
PvP East 1
PvP East 2
In terms of population, the PvP West and PvE East clusters will likely be the largest due to the free transfers they received. The other, smaller clusters host some dedicated communities. RP realms were exempted from realm connections. The RP-PvP server of Bloodsail Buckaneers is currently hosting a large portion of the ‘Hardcore’ ruleset community.