You can delete Products from your SI Catalog individually or you can delete multiple Products at one time using the Delete button. You can select multiple Products in the grid by holding down Shift (to select a series) or Ctrl (to select outside of a series).
Deleting a Manufacturer will delete all Products from your Catalog assigned to the Manufacturer.
Deleting a Category will delete all Products from your Catalog assigned to the Category.
There is no "undo" in SI. Once deleted, there is no recovering these Products unless you restore a backup of your SI server.