Yes, enterprise value (EV) is equity market capitalization (stock price times # of shares) plus the market value of debt (this can be lower than the book value of debt). Financial analysts often prefer to calculate something called "Net Enterprise Value", which is EV minus "excess cash" (total cash minus "required cash", where required cash is usually calculated as a certain % of sales). Net Enterprise Value gives you the present value of all future cash flows to the firm (you are ignoring the value of today's excess cash, which doesn't come from future cash flows). This is particularly useful when looking at companies like AAPL with tons of cash.

As mentioned below, enterprise value is generally calculated as equity value + debt - (excess) cash. Ebay has approximately $5 billion more cash than debt, so its EV is less than its market cap.

One way to think about why cash is subtracted is that the acquirer gets to keep it. If I pay $10 billion for 100% of a company's shares, but get to keep the $1 billion in cash on the balance sheet, then the actual price of acquiring the company is only $9 billion (assuming no debt). With enough cash, it is possible to have a negative enterprise value.

EV is usually calculated with the current equity value, as that is what the market "believes" the company is worth. If an acquirer comes along and wants to purchase all of the shares at a premium, you can find an implied EV from the offer price.