Retained earnings is an accumulated account for year to year turn over.
When you close a fiscal year the profit and loss for the year is debited the Profit/Loss Year and credited the Retained Earnings. This is if it was a profit. The other way round if it was a loss.
The Sales Account is the account that is credited every time you invoice your customers.
If you are interested in Accounting, there are links to good instructions here:
/Joe