Six sources: Martial, Divine, Primal, Arcane, Shadow, Psionic. Simple enough.
Four specializations, meant purely as descriptive for the baseline of a class, something you build up or out from. Defense, Offense, Support, Control. Again, simple enough.
Within them, focuses (again, purely descriptive, build toward what you want if it isn't the starting focus). Offense specialists can start with a "Skirmisher" focus (fragile speedster, zips in, clobbers, and zips out) or "Bruiser" focus (durable/self-sustaining wrecking balls.) Defense specialists can start with a "Bulwark" focus (sky-high defense and enemy/location lockdown) or "Slayer" focus (strong offense and defense and brutal counter-attacks). Etc.
By level 10, any character can pick up a second focus, whether in their specialization or outside of it, with a bit of effort (e.g. 1-3 feats, equipment useful to that focus, and appropriate powers), especially if they choose a subclass that already minors in some other specialty. Any character, by level 13-15, should be either a total master of their original focus, or very good at both that focus and a second that meshes with their subclass, e.g. a War Wizard (Control, focus Warder) could pick up Defense specialist stuff (e.g. invoking AC-boosting actions, using a Staff of Defense spellcasting focus, taking the Polearm-Warrior and Arcane Vitality feats), becoming a shockingly durable spellcaster able to lock down foes near and far and shrug off blows that really should kill someone just wearing a bathrobe. Any character by level 20 should be able to pick up any second focus they like, whether it is a natural fit for their class/subclass or not, it just requires different (and likely greater) investment. In theory, a level 20+ character could even try to add a third focus, but this would be stretching other parts of the game--like the action economy--to their limits.
Classes (and their specialties)
Martial: Fighter (defense, Slayer), Rogue (offense, Skirmisher), Ranger (offense, Bruiser), Warlord (support, Vanguard), Scout (control, Warder). All Martial classes have a natural bias toward offense specializations.
Divine: Paladin (defense, Bulwark), Avenger (offense, Skirmisher or Bruiser), Invoker (control, Guide), Cleric (support, Mender). All Divine classes have a natural support bias.
Primal: Warden (defense, Bulwark), Barbarian (offense, Bruiser), Druid (control, Warder), Shaman (support, Vanguard or Mender). All Primal classes have a natural defense bias.
Arcane: Swordmage (defense, Slayer), Sorcerer (offense, Bruiser or Skirmisher), Wizard (control, Warder or Guide), Bard (support, Mender or Vanguard). All Arcane classes have a natural control bias.
Shadow: Ninja (defense, Slayer), Assassin (offense, Skirmisher), Warlock (control, Guide), Occultist (support, Vanguard). Shadow classes do not have a natural bias; instead, they fulfill their specialization in unique or unusual ways, e.g. Ninjas use stealth and guile to avoid ever taking damage in the first place, and Occultists grant temporary benefits that must be locked in through violent conflict.
Psionic: Monk (offense/defense, Skirmisher) and Psion (control/support). The Psionic source is unique; its classes are looser by design, allowing the player to mix and match parts as they desire, with no guarantee of functionality. "Monk" and "Psion" can even dip from one another's pools for the price of a single feat, allowing almost total freedom to assemble whatever character the player likes.