The exhaust manifold is the first collector system for the overall exhaust system, and is also the hottest part of the system when the engine has run for a while. They may combine all at once (at a single point) or gradually collect in sets, depending on the design and room available.
High Performance Headers
Headers are a popular upgrade option due to being relatively easy to install them (the engine don't have to be taken apart) and the low cost-per-horsepower gains.
Exhaust Headers Explained by Engineering Explained
In this video, Jason explains how a header works.