When you start planning more than day trips on your motorcycle, where to carry things starts to be an issue. Tents, sleeping bags, or even clean clothes and a toothbrush have to go somewhere.
If you are alone on the bike and are looking for something simple you can buy an inexpensive duffel bag, stuff it full and strap it behind you. A simple system, this might be enough for many happy trips. But you may eventually want to go on a longer journey, or need more equipment, or just want to bring someone else along on the back where the duffel had been strapped.
You may have purchased a motorcycle with integral luggage. Many modern touring machines come with sidebags included and higher end models (such as Honda's Goldwing) include a truck and enough space for weeks of travel if packed sensibly. However, smaller (and less expensive) motorcycles will not have these, and with the increased popularity of adventure touring motorcycles (such as BMW's GS line) numerous aftermarket luggage systems have appeared on the market.
Most motorcycles still come without a luggage system, though the increasing popularity of motorcycle travel means more and more entry level bikes have at least some sort of luggage as an option. When considering this option you should pay careful attention to the cost, as aftermarket bags might be a more cost-effective choice.
If you are shopping for luggage the biggest choice you will have to make is whether to use hard of soft sided bags. Many overland adventure bikes have aluminum sided boxes which can be mounted on racks to the sides, and this is a popular option. While many people use these without complaint, others comment on the hard sides causing problems in tip overs, with some riders even reporting the hard edges of the bags causing broken legs. Less seriously, due to the aluminum construction, repairs can be difficult in some areas if the boxes crack. Other boxes are made from lightweight alloys, where repairs become even more difficult. If you are planning to remain inside the United States or any other developed country this will probably never be an issue.
Hard sided bags do have many benefits. They either come with, or can be fitted with, locks for increased security. Because they are sturdily built they can protect the sides of the motorcycle in the event of a crash, and of course they make the perfect place to display stickers of all the places you've been.
Soft bags, usually of a type that are thrown over the rear of the bike, are usually much less expensive than their hardsided cousins. A set of soft bags from Ortlieb will cost under $200, which won't even buy one side box from Touratech, a maker of hard sided bags. Hard bags also require a rack to mount them, adding to the cost. Many soft bags are not waterproof (though the Ortlieb bags mentioned above are), and may tear in the event of a crash. Care must be taken on some motorcycles to keep them from contacting the exhaust, which can cause melting or burning. And because Plano fishing tackle bag are soft, if the bike does fall over they don't provide much protection to the bike, or the contents of the bag. When traveling with softbags I am careful not to put anything too fragile inside them, but even this didn't stop a can of bear spray from rupturing and coating everything side.
Whether you choose soft or hard bags is ultimately a matter of personal choice, and I find myself switching from one to the other depending on my needs and destination. It is more important to choose what makes you comfortable and then use them to actually go out and travel, than to spend months or years trying one or the other and never actually go anywhere.