pointers, assignments, Garbage Collection Oh My?

[JohnnyK]

I hope you like the subject matter and I hope it is not too 
simplistic or have been answered before.
   Anyway I have a question about how the garbage collector works 
in a very specific situation.  When passing string type to a 
function in a shared library or DLL and assigning it to a 
variable of type string inside the function and returning the 
internal string.  Such as this.

export string mytest(string tstStr)
{
   string st = tstStr;
   /* abbreviated to protect the innocent but other operations
      such as concatenating and deleting may be done to st before 
the return
   */
   return st;
}

Is the string type a pointer or is it something else?  In the 
line where tstStr is assigned to st does it copy the address in 
tstStr to st or does it copy the value in tstStr?  I am just a 
bit confused about string types since I come from a C background 
and C has no type like this.  Also what is returned by this 
function?  Does this function return a pointer or the contents of 
an array?  If I do export this what does it do to the Garbage 
Collection?  Does the Garbage Collection collect tstStr or st?  
Also notice the comment in the function.