As the name suggests Selling, general and administrative expense (SG&A) refers to the expense incurred in administering a business as well as selling the products / services created by it.
General & administrative cost includes all expenses incurred in operating a business other than the cost directly incurred in producing the product sold or services rendered. Items like salaries of top executives & office staff, rent of office building, auditor fee etc. are part of this head.
Selling expense includes the cost of advertising and other promotional expense.