Yes, you can get them on both petrol and diesel engines.
When you're using a turbocharger it is compressing the incoming air. Basic thermodynamics causes the temperature of the air to rise, and as we all know this is bad for performance.
An intercooler is effectively an air-cooled matrix, that the compressed air from the turbo passes through before going into the engine. Air rushing over the radiator removes heat from the intake charge, cooling it down and increasing performance.
You can get water cooled and dry ice cooled intercoolers too, or spray them with a mist of water or N20 to reduce their temperature.
On average, the bigger the intercooler, the more chance you stand of dropping the intake air temperature more, allowing for higher performance (or more boost, if you're heating it up so much that performance is falling)
No need to apologise for the dumbness! Makes you look more sensible if you ask instead of rambling or being uncertain! Reminds me of the time when my mate spent half an hour lecturing some friends on how a DUMP VALVE was a section of the exhaust that got switched over when you changed gear, which made the noise

....I laffed, and let him continue
