How exactly does __attribute__((constructor)) work?

It seems pretty clear that it is supposed to set things up.

1. When exactly does it run?
2. Why are there two parentheses?
3. Is __attribute__ a function? A macro? Syntax?
4. Does this work in C? C++?
5. Does the function it works with need to be static?
6. When does __attribute__((destructor)) run?

Example in Objective-C:

__attribute__((constructor))
static void initialize_navigationBarImages() {
  navigationBarImages = [[NSMutableDictionary alloc] init];
}

__attribute__((destructor))
static void destroy_navigationBarImages() {
  [navigationBarImages release];
}