Headers = aftermarket tubular steel manifolds.

Manifold = that cast iron piece of shit attached to your stock exhaust.

Granted, some cars come with tubular steel manifolds from the factory, I know VW is doing that on their base model 2.0 engine in the MkIV and I have seen them on Toyotas and Hyundais/Kias too. But you will never see them called "headers" in any technical manuals or literature put out by the company.

I'll look this up in the OED when I have access at work on Monday, but I am 99.9999999% sure that an original equipment manifold is never referred to as a "header" in proper English.

Also, people are probably only calling it "headers" the same way they erroneously pluralize "Krogers" "Targets" and "Sonics". Its just that they're Southern and ignr'nt.