Blocks are predefined places in themes for placing widgets in them. In the Blocks administration page, the yellow box displays available widgets. You can move widgets to blocks using drag and drop. Also each widget has visibility options for different languages and roles under flag icon in widget box. You can choose where to see and who to see widget by clicking on the flag icon and changing this option. Moreover some widgets depended on the programming could have a gear icon beside flag icon that shows customization options for that widget.