There is no risk to a stock option until you decide to purchase the stock. You normally buy a stock with cash. When you buy the stock, you have exercised your option. Hopefully you can exercise the option to buy when the stock has risen in value above the option price. While the exercise period sets the last possible buy date, the vesting period sets the initial date at which you can exercise part or all of your option.
Some firms require a one-year vesting period before any shares can be purchased.
Others phase in the vesting period in annual percentages, allowing portions to be exercised each year until full vesting is reached. Option overhang is the term used to describe vested options that haven't been exercised. Many firms will offer additional incentives, such as bonuses, to get employees to exercise their stock options in a certain time frame. In this high-tech age, when stock options are prevalent as a way for firms to hold on to cash, it's not unusual for employees to become rich on stock options. Be careful not to tie your fortune in too closely with your employer's company.