Working capital is defined as current assets less current liabilities. Current assets are inventory, receivables and cash, while current liabilities are amounts owed to suppliers, bank overdraft and other short term liabilities such as taxes due.
Managing working capital is very important. Tie up too much money in inventory and the business is in trouble. A recent report by PWC suggests companies are still not managing working capital as best they can. Read about it and some suggestions to improve working capital here.