In most production engines, the manifold is an assembly designed to collect the exhaust gas from two or more cylinders into one pipe. Manifolds are often made of cast iron in stock production cars, and may have material-saving design features such as to use the least metal, to occupy the least space necessary, or have the lowest production cost. These design restrictions often result in a design that is cost effective but that does not allow the engine to breathe efficiently. Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates a back pressure and restriction in the engine's exhaust system that can restrict the engine's true performance possibilities.
A header is a manifold specifically designed for performance. During design engineers create a manifold without weight or cost in mind, but instead for optimal flow of the exhaust. This design results in a header that flows much more efficiently than stock exhaust manifolds. Headers are generally circular steel tubing with bends engineered to make the exhaust flow from each cylinder's exhaust port to the common outlet all equal length, and joined at narrow passage ways to encourage pressure to flow through the outlet and not back towards other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow to be matched within and rpm range.
Headers are generally made by aftermarket automotive companies, but sometimes can be bought from the high-performance parts department at car dealerships. Generally, most car performance enthusiasts buy aftermarket headers made by companies solely focused on producing reliable, cost-effective well-designed headers specifically for their car. Headers can also be custom designed by a custom shop. Due to the advanced materials that some aftermarket headers are made of, this can be expensive. Luckily, an exhaust system can be custom built for any car, and generally is not specific to the car's motor or design except for needing to properly connect solidly to the engine. This is usually accomplished by correct sizing in the design stage, and selecting a proper gasket type and size for the engine.
Most instances replacing stock exhaust manifolds with aftermarket headers is not only better for your engine but better for your wallet as well. Factory replacements can easily equal out to the cost of a nice set of stainless headers. Another thing to consider is the life and longevity of the parts you are putting on, typically if your manifolds on your 07 pickup came off looking like swiss cheese with 56,000 miles on them it will happen again using the same exact parts. If you're planning on keeping the vehicle why not upgrade to a nice set of stainless headers that will in most cases out live the vehicle, Not to mention the increase in horsepower, torque, and fuel economy. Most factory replacements also carry little to no warranty as opposed to say Gibson headers which carry a lifetime limited warranty! Either way you choose to go when it comes to manifold leaks don't waste too much time, the leak can be bad for your engine and the noxious fumes are bad for you!