No. ‘Clusters’ is a community-derived term used to denote the groups of realms that have had Realm Connections (NOT Cross-realm - that is something altogether different) implemented. Realm connections are Blizzard’s version of a merge. They are permanent links where all players in a given cluster can group/guild/raid/trade/same AH/etc. with each other at all times. A list of the North American/OCE clusters for Classic Era can be found in this bluepost:
Layers are a technology used when a given ZONE has too many players in it at one time. These are spun out and collapsed dynamically as a way to maintain server stability.