In views, what is the difference between Normal menu entry, Menu tab and Default menu tab
They correspond to the type parameter of items returned from hook_menu()


Normal menu entry = MENU_NORMAL_ITEM


  Menu type -- A "normal" menu item that's shown in menu and breadcrumbs.
  
  Normal menu items show up in the menu tree and can be moved/hidden by the administrator. Use this for most menu items. It is the default value if no menu item type is specified.

Menu tab = MENU_LOCAL_TASK



  Menu type -- A task specific to the parent item, usually rendered as a tab.
  Local tasks are menu items that describe actions to be performed on their parent item. An example is the path "node/52/edit", which performs the "edit" task on "node/52".
  Default menu tab = MENU_DEFAULT_LOCAL_TASK



  Menu type -- The "default" local task, which is initially active.
  
  Every set of local tasks should provide one "default" task, that links to the same path as its parent when clicked.



The best example I can think of are node pages. When you visit a node page, the 'View' tab is provided by a menu item of type MENU_DEFAULT_LOCAL_TASK (see $items['node/%node/view'] in node_menu()). 

It doesn't have a path as such (it inherits from the parent menu item), it's only purpose it to allow the node page to have a tab amongst the groups of 'real' tabs, which are defined with MENU_LOCAL_TASK.