Okay first of a great many thanks for taking the time out to give me such a detailed explanation. I could'nt have got better. But I have a couple of questions.
Are you saying that for example I have a piece of source code like -:

Code:

#include<stdio.h>
main()
{
printf("Hello World"\n");
}

And I compile it on a SCO UNIX machine and take the executable to Solaris Machine the executable won't run. Are you saying that I would need the source of the hello world program and then I would have to build it back on the Solaris Machine ?

I am sorry but could you elaborate what you mean by this. The kernel is at the end responsible for how software interacts with hardware, so it kinda does everything.

One last question it may be off-topic. You said -:
How do I contact someone like Don Cragun ? I am not saying that your answers were wrong or insufficient in any way but if I wanted to contact him then how would I do it ? By private message ?

Yes it did immensely.