exhaust back pressure?

I have read and heard many things about back pressure and just don't get it. I know it concerns the exhaust and having a muffler and cats, but what does it do? I've also heard people say that no back pressure will causse gas to leak out the pipes. Is this true? If so, is there a difference in amount between carburated and fuel injected?

To keep this post shorter,i will simplify.When you put your thumb over a waterhose,it causes backpressure,cause the amount of water flowing into the hose is greater than your thumb will allow it to exit the end.The harder you press your thumb over the hose,the more pressure,and less flow you will have.The water hose becomes harder the more flow you restrict,cause the added pressure the restriction creates.So the pressure increases,as the flow decreases.Turbo cars run on backpressure,cause the extra pressure in the exhaust is what is used to drive the turbo.When you see guys with a turbo saying they are running *X* amount of psi boost,that means they have that amount of backpressure in psi,in their exhaust system,wich in turn causes the turbo to work better.But a fine line exists between helping and hendering.Too much backpressure can cause blown seals,and other problems as well>my car has a regular carb on it,and backpressure is a cuss word for a car with big valves,and no turbo

my explanation is a little rough,but it kinda gives you a basic idea of how backpressure works.

thanks, that helps alot