Someone (maybe Joseph Kennedy) said the flip of this, namely: "when the bellhop in the hotel gives you a stock tip, it is time to sell".
A bull market is one where prices are increasing and investors are confident and optimistic.
A bear market is one where prices are decreasing and investors are fearful and pessimistic.
Selling "short" is when you predict that prices will fall, so you sell something now, expecting to be able to buy it later at a lower price. This makes you money when the thing you are selling is borrowed from a third party, and you make enough money to also repay the lending fees.
Selling long is the opposite (and more conventional), where you predict that prices will rise, and simply hold on to the commodity.
A put option is a purchased right to sell some "instrument" to the seller of the option at an agreed upon price, during some agreed upon interval of time. Someone will purchase a put option to acquire a short position. Not that the option need not be exercised, but the person selling the option must buy the instrument if the option holder chooses to sell it.
A call option is a purchased option to buy shares of stock at some specified time interval in the future at an agreed upon price. Note that the buyer may choose to not exercise the option, but the seller must honor his promise to sell the stock if the buyer wants to buy. Essentially the person buying a call option is gambling that the price of the instrument will rise, whereas the seller of the option is gambling that it will fall. The agreed upon price is called the "strike price".
If the price does rise sufficiently for the holder of the call option to buy the stock, recover enough to cover the cost of the option and also sell the stock at a profit, they will exercise the option. If the stock price falls (or holds steady), they will not exercise the option, and the option seller will have gained the cost of the option.