Noun: The opposite of a put, a call is an option that gives you the right to buy a given stock (or commodity or other asset) at a given price in a given period. You pay a fee for this privilege. For instance, you might buy some IBM calls giving you the right to buy IBM shares at $125 by a certain date. If the stock rises above $125, you make money. If it doesn't, the call expires worthless, and you're out the fee you paid. Calls are often used to hedge risk. Covered calls appeal to conservative investors.
Verb: A bond issuer might have the right to call a bond, meaning redeem it early, usually because interest rates have fallen.