Hello
I have some difficulty in understanding the following

a) Normally apex code & trigger run in system.context
   But How to make the apex code execute in USER Mode?
   if the apex code executing in USER Mode  is called by a trigger, will the trigger also execute in USER Mode?

b) Can a visual force page run in USER Mode instead of system.context , again what changes need to be done and why?

c) By default webservice classes written in apex , rest api execute in what Mode?

d) what is the use of system.RunAs with respect to the above.

I hope I am clear

Thanks
Vandana R