CSS concept...
Different between id and class:

      Ids are to be used only once in your HTML layout.

    Classes can be used multiple times. Ids have priority over classes. So, if an element has a both id and class, the id will take over.

     Sample: Id: #id_name {}     class: .id_name{}

·  Different between padding and margin: 

     Padding is the space inside the border between the border and the actual image or contents.
     Margins are the spaces outside the border, between the border and the other elements.