I would say comping means choosing the best results from different and many recordings - takes - and taking the best ones. For example Mark can sing his vocal part several times and the final choice then is built, combined, - or comped - from these recordings... (I remember some discussion some years ago that you could clearly hear on the the track "The Ragpicker's Dream" where the different vocal parts were ... I listened a dozen times but was not able to make out where the differences were..
) So different guitar parts, different drum parts and so on, listening to them in a row the whole day (which is what they did obviously on the famous Ragpicker diary picture on the London roof in the sun, you remember) - taking final decisions, discussing...
mixing then is purely a question of mixing the final tracks together, volume, fades, dynamics and stuff.
Any better explanations welcome!
LE