When to use brackets and when to use the period in Objective-C

I'm a new iPhone/Objective-C developer and as I'm going through different tutorials and open source code, I am having a bit of a problem understanding when to use the square brackets "[ ]" and when to use the period " . " for accessing properties/methods of an object.

For example, this code:

- (void)setSelected:(BOOL)selected animated:(BOOL)animated {

  [super setSelected:selected animated:animated];

  UIColor *backgroundColor = nil;
  if (selected){
    backgroundColor = [UIColor clearColor];
  } else {
    backgroundColor = [UIColor whiteColor];
  }

  self.todoTextLabel.backgroundColor = backgroundColor;
  self.todoTextLabel.highlighted = selected;
  self.todoTextLabel.opaque = !selected;

  self.todoPriorityLabel.backgroundColor = backgroundColor;
  self.todoPriorityLabel.highlighted = selected;
  self.todoPriorityLabel.opaque = !selected;
}

Why does [UIColor clearColor] get brackets, but todoTextLabel.backgroundColor get the period?

Could someone explain this easily for me?