Hi.

The Sale_order is the base order, model 'sale.order'

The 'sale.order.line' is a joined model (table) what is contain the order relevant items. (product ids, quas, and so on)

You can see the sale_order() columns definition, the sale.order.line is a one2many field of sale_order that is a connection betwwen the two table.

'order_line': fields.one2many('sale.order.line', 'order_id', 'Order Lines', readonly=True, states={'draft': [('readonly', False)]}),

Hi Kit ,

Sale_order is the Master / Header object for the Sales / CRM module 

and Sale_order_line , is the Individual line items in a Sales order , i.e. the product and services

with price and quantity you add in to a sales order. 

Hope this help !