Hi everyone

I am trying to learn things about my car without having the modifications done on my car. Basically I'm here only to learn.

I was wondering what the use of a header is. Does it increase HP? How much does it cost? How easy or hard is it to do the install myself or what would the labor cost? Is it a good investment? What are the effects on others things like fuel efficiency or engine sounds. How often should i get one/ how fast does it rust? Which companies are good, which should I stay away from?

any information imagineable would be deeply appreciated, thanks in advance

p.s. I have a 98 acura EL