In finance, a put is an option contract that gives the owner (the option holder) the right (but not the obligation) to sell a specified amount of stock (or other security) at a specific price on a certain date.
For example, you might buy from me a put (a guarantee) that I would buy 100 shares of Alltel stock from you at $50 each on June 15th -- if you wanted me to. I would then be under a legal obligation to buy these stocks from you even if Alltel went bankrupt. On the other hand, if Alltel goes up in price to over $50, you keep your shares, and I have made a small profit (the price you paid me for the put). It's basically stock insurance.
A put may be backed up with a guarantee letter.
The opposite of a put is a call, where you buy a guarantee that the other party will sell a certain amount of whatever to you.
There is also a tricky thing called a synthetic put, in which you both sell short and purchase a call option. This basically works out the same as a put, in a twisted sort of way.
Some bonds may have a put option (these are called put bonds, unsurprisingly). In these cases, a long-term bond can be sold at certain specified dates before it matures. The pay-out will generally be scaled down as if you were cashing in an equivalent short-term bond.
This is not quite the same as the put you buy for stocks or commodities, because you are not buying an option from a third party. The put option is set up from the start by the issuer of the bond.