1 - What does it actually mean that the sound/sample is 8-bit ?
Reading the wikipedia article about
pulse-code modulation should help here. In short, 8-bit samples in your usual tracker are PCM samples, and a resolution of 8-bit says that there are 256 (2^8) discrete quantization levels (which is not a whole lot).
2 - The sound on old computers was generated using different sound chips. How did they differ as far as the sound generation is concerned ?
Well, many of them (f.e. the SID chip in the C64) were synthesizers - they could only generate sound from a given set of waveforms (square, triangle, etc) - some of them have been abused to output PCM (any desireable waveform) as well, too. The Amiga's Paula chip on the other hand is designed to output digitized (PCM) audio. Most chips differed in the number of channels / waveforms available, and many had a distinct sound due to that.
3 - Does Milky emulate any of these sound chips ?
Milkytracker can emulate the resampling characteristincs of some Amiga models which applied some kind of lowpass filter for smoother audio (so it didn't sound too "raw"), which can be applied from the I/O settings in the resampling section. Dunno about any other emulated characteristics, an actual milky user might have more information available.
4 - What is the difference between for instance drum sample generated in Milky and some real drum hit recorded and imported as a wav file ? Is it possible to recreate that 'real' sound drawing exactly the same wave shape in sample editor ?
You will never be able to recreate most "real-world sounds" from manual sample drawing - those are simply too detailed to be reproduced by hand. Other than that, if you were possible to recreate the original waveform sampling point by sampling point, there would be no difference of course.