Book Value is calculate as Total Assets mins Total Liabilities. It is a measure of Shareholder's Equity.
Book Value - or Net Asset Value - is a measure of shareholder's equity and is calculated as Assets less Liabilities and any Preferred Stock.
From the accounting perspective, it could be seen as the underlying value of a company, whereas the market capitalisation value is dictated by the supply and demand of shares.
However, should the company go into liquidation, the Book Value is often used as a proxy for the value remaining for common shareholders after all creditors are paid.