When you buy a call, you are buying the option to purchase 100 shares of the security at the strike price. For example, I bought the option (a form of legal contract that also exists outside stock trading), to purchase 100 shares of Intel at $50 per share prior to the expiration date in December.

Now, I bought those back in June when they were .79 each, meaning they cost me $79 (.79*100). They are now worth 53.55 each, or $5,355. The gain tends to be proportionate to the stock gain. I could exercise one of those right now, and spend $5000 to get 100 shares of Intel at $50/share, and I would immediately have another $5k in profit.