Makes the text bold that is between the start and end tags. Makes the text Italic that is between the start and end tags. Makes the text Underlined that is between the start and end tags. It creates a 'line' across the screen. A separator for information.
Size is the 'thickness' of the hr.
Default is 1. Width is again simple and is just how wide it is. You can put a number in and it would be the same no matter what the browser. It just makes it a percentage of the page. Noshade which actually means ' No Shade', just takes away the 3D look and makes it one color.