Cart's answer describes well one aspects of puts: protective puts; which means using puts as insurance against a decline in the price of shares that you own. That's a popular use of puts. But I think the wording of your question is angling for another strategy: Writing puts.
Consider: Cart's strategy refers to the buyer of a put. But, on the transaction's other side is a seller of the put – and ultimately somebody created or wrote that put contract in the first place! That first seller of the put – that is, the seller that isn't just selling one they themselves bought – is the put writer.
When you write a put, you are taking on the obligation to buy the other side's stock at the put exercise price if the stock price falls below that exercise price by the expiry date. For taking on the obligation, you receive a premium, like how an insurance company charges a premium to insure against a loss.
Example: Imagine ABC Co. stock is trading at $25.00. You write a put contract agreeing to buy 100 shares of ABC at $20.00 per share (the exercise price) by a given expiration date. Say you receive $2.00/share premium from the put buyer. You now have the obligation to purchase the shares from the put buyer in the event they are below $20.00 per share when the option expires – or, technically any time before then, if the buyer chooses to exercise the option early.
Assuming no early assignment, one of two things will happen at the option expiration date:
ABC trades at or above $20.00 per share. In this case, the put option will expire worthless in the hands of the put buyer. You will have pocketed the $200 and be absolved from your obligation. This case, where ABC trades above the exercise price, is the maximum profit potential.
ABC trades below $20.00 per share. In this case, the put option will be assigned and you'll need to fork over $2000 to the put buyer in exchange for his 100 ABC shares. If those shares are worth less than $18.00 in the market, then you've suffered a loss to the extent they are below that price (times 100), because remember – you pocketed $200 premium in the first place. If the shares are between $18.00 to $20.00, you're still profitable, but not to the full extent of the premium received.
You can see that by having written a put it's possible to acquire ABC stock at a price lower than the market price – because you received some premium in the process of writing your put. If you don't "succeed" in acquiring shares on your first write (because the shares didn't get below the exercise price), you can continue to write puts and collect premium until you do get assigned.
I have read the book "Money for Nothing (And Your Stocks for FREE!)" by Canadian author Derek Foster. Despite the flashy title, the book essentially describes Derek's strategy for writing puts against dividend-paying value stocks he would love to own. Derek picks quality companies that pay a dividend, and uses put writing to get in at lower-than-market prices. Four Pillars reviewed the book and interviewed Derek Foster: Money for Nothing: Book Review and Interview with Derek Foster.
Writing puts entails risk. If the stock price drops to zero then you'll end up paying the put exercise price to acquire worthless shares! So your down-side can easily be multiples of the premium collected. Don't do this until and unless you understand exactly how this works. It's advanced.
Note also that your broker isn't likely to permit you to write puts without having sufficient cash or margin in your account to cover the case where you are forced to buy the stock. You're better off having cash to secure your put buys, otherwise you may be forced into leverage (borrowing) when assigned.
The Montreal Exchange options guide (PDF) that Cart already linked to is an excellent free resource for learning about options. Refer to page 39, "Writing secured put options", for the strategy above.
Other major options exchanges and organizations also provide high-quality free learning material: