Core D&D books have always deliberately stayed silent on the issue. (Get it? Silent? I crack myself up.) In Pathfinder, for example, the rules just say
A verbal component is a spoken incantation. To provide a verbal component, you must be able to speak in a strong voice. A silence spell or a gag spoils the incantation (and thus the spell). a spellcaster who has been deafened has a 20% chance of spoiling any spell with a verbal component that he tries to cast.
It doesn't specify language, format, length, etc.
Probably the most famous example of D&D verbal components, however, is the mage Raistlin from the Dragonlance novels, who always cast Light with the word Shirak. Go here to see a relevant quote from The Soulforge. There's some debate over whether that's a spell cast or a spell activation command word for his staff, but he also casts Sleep on someone with the words "Ast tasarak sinularan kyrnawi." That's their approach; most other gaming fiction BSes it away as "arcane syllables roll off the wizard's tongue..."
In general D&D tries to leave this undefined so that if in our campaign world you want arcane words that's fine, or if you want incantations or whatnot like the anime Slayers you can do that ("Darkness beyond twilight. Crimson beyond blood that flows..."), or Ars Magica real/foreign spell words ("Perdo Ignem!").
So the real answer to this is 'decide how you want it to work in your campaign!'
Having said that, a more recent example of "what spellcasting works like" is available in the Pathfinder comics (overseen by Paizo and some written by Paizo employees). There they appear to have made the artistic decision to use several of these models, the "weird words" model for arcane spellcasters and the "Slayers" model for clerics. See these panels starring the Pathfinder iconic NPCs: