A “stock option” is the right to make a transaction, buy or sell, a stock at a defined price within a specified period of time. For an employee, “stock options” differ on the basis of maturity: your stocks “vest” as opposed to the option to buy/sell expiring within a specified period of time.
Stock Option selection on Investopedia
Richard Harroch's How Employee Stock Options Work in Startup Companies at Forbes
Alyson Shontell's If You Want To Get Rich At A Startup, You'd Better Ask These Questions Before Accepting The Job at Business Insider
Scott Kupor's How Startup Options (and Ownership) Works at Andreessen Horowitz
Andy Rachelff's The Right Way to Grant Equity to Your Employees at First Round Review
Babak Nivi's I have a job offer at a startup, am I getting a good deal? Part 1: The Offer at Venture Hacks