"headers" (american term) the pipes which come out from the head of your car (exhaust side)
"Exhaust manifold" Correct term for Headers (but also loosely used for a complete system - front to back)
"Downpipe" the curved part of your manifold where your pipes all join together.
Standard (4cyl) cars go form 4 to 1 pipe (not always eqal length in the past)
performance (4cyl) cars have a 4 to 2 to 1 setup (reffered to as 4-2-1) and they are all equal length and diameter.
If i'm wrong I'm sure RacerF1 or FLi_GTi will point out my mistakes.