The Click event was originally defined in Silverlight for desktop Windows and it is only defined for the Button control (and derivatives such as HyperlinkButton). You can think of the Click event as the "traditional" way to handle a Button press.
The Tap event was added to the framework in Windows Phone 7.1 (Mango). Tap is defined in the UIElement class, which is the parent of many types of controls. You can handle a Tap event in a TextBlock, Image, and many other controls. Button is subclassed from UIElement as well, and thus can also receive Tap events. It is redundant that a Button can receive both Tap and Click events.