Officially, though it took a long time for it to shake out*, most Essentials and post-Essentials classes are 'subclasses.' They're distinct from classes and builds. Builds have probably been abandoned, though Warpriest Domains and Mage schools come awefully close.
A number of pre-Essentials classes have be re-cast as sub-classes, so the pre-Essentials Fighter is now the 'Weaponmaster' sub-class (yes, even the Brawler build), the pre-E Cleric is now the 'Templar,' and so forth. Not /every/ class has gotten this treatment, though, the Warlock somehow escaped, for instance, and, AFAIK, no PH2 or 3 classes have been demoted as yet.
In summary:
4e Heirarchy: Source > Role > Class > Build
Essentials+ Heirarchy: Class > Subclass > Source(s)|Role(s) > Domain|School|Build?
WotC finally bit the bullet in the 'Class Compendium,' that was ultimately cut up and published in Dragon. Subclass is defined here:
http://www.wizards.com/dnd/files/CC_Feats.pdf
"Many classes encompass one or more subclasses to demonstrate different expressions of the class concept."
FWIW