Rects are generally represented as a left, top, right and bottom coordinate. However there are other methods of representing them that may be more useful depending on the information you have at hand.
For instance you may know the top left position, the width and the height of your Rect. To turn this into a Rect you would have to work out the bottom right corner by adding the width and height onto the top left position.
Or you may know the top left and bottom right positions but want to move the Rect down and across by a few pixels. Or again you may know the top left position and just want the rest of the rectangle to fill whatever is left of the Canvas.
You can work out all of these quite easily but to save you time a number of facilities exist to work out what you want automatically from the information you provide.
You can specify a Rect in a number of other ways. For instance you can specify:
If no information is provided the Rect is assumed to fill the Canvas. If only a position is provided the Rect is assumed to be offset by this position. If only a size is provided we assume the position is at 0,0.