right, i0ve got a really stupi nooby question.

when you have an amp and a distorter, are you getting the distortion from the amp or the pedal.
for example, let's say i have a marshall jcm series, and a boss distortion pedal, is the distortion the marshall amps distortion or is it given by the pedal???
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It depends what channel your amp is on, if you have any gain on the amp, and weather you have the pedal turned on.
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a distortion pedal gives the distortion, generally its used on the clean channel of your amp so the distortion is only coming from the pedal

when using a pedal to boost (usually overdrive pedals) you use your amp on the overdriven channel and modify the distorted sound with the pedal

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with a jcm800, I'd say you'd use the amp's distortion, because it's going to be a great distortion. Generally speaking, tube amps have a great overdrive, however sometimes, the amp won't be able to give you enough distortion if you play heavier genres, in which cases you'd use a pedal
If you turn the gain up on the pedal to get distortion, its coming from the pedal.

If you turn the level up on the pedal and the gain down to get some grit, its coming from the amp.
it can be one, the other, or both. if you have gain up on the amp and down on the pedal, its the amp. if you have the gain up on the pedal and down on the amp, its the pedal. if you have the gain up on the pedal and the amp, it is both.

or if you have the level up on the pedal and the gain sorta up on the amp, you can cause the amp to distort more by using the pedal as a boost.