Working capital is the difference between current assets and current liabilities, measuring short-term liquidity and ability to meet operational obligations. Current assets include cash, A/R, inventory, and short-term investments; current liabilities include A/P, accrued expenses, deferred revenue, and short-term debt. Positive working capital means more current assets than current liabilities; negative working capital is normal for some business models (subscription companies with annual billing) and concerning for others (companies that owe more than they can collect).
The math:
Working capital = Current assets - Current liabilities
Current assets include: