what are the differences between transaction body field and entity field ?

Generally,
Transaction Body and Entity fields are field type classifications used in the system to determine its specific record application .
In NetSuite, customers, vendors, partners, employees, contacts, etc. are referred to as entity records.
Any field whether standard or custom that is applied/displayed on these records types are called entity fields.
On the other hand, an example of transaction records are Sales Orders, Invoices, Vendor Bills, Purchase Orders, Item Fulfillment, etc. There are (2) field conventions in a transaction record.
These are the transaction body and column fields.
column fields are those which are in Line Item sublist . Rest fields are called body field.