There are many styles of motorcycle panniers suited to many different uses. Touring panniers are usually are sold in pairs, and larger models are intended to hold enough equipment for self-sustained tours over days or weeks, including sleeping gear, food, and cooking equipment. Touring panniers tend to be the largest type of pannier and are often designed to hold more than 50 litres each. Panniers come in both hard and soft forms and there are dozens of manufacturers producing every variation you can imagine.
If you are strapped for cash and reasonably handy, you could make up your own. There is an interesting post at ADVrider showing how someone made up their own panniers. The serious rider touring around the world generally has aluminum panniers that are lightweight, survive when a bike is dropped and are secure.
I am not recommending any of the motorcycle panniers on this page; they are simply to illustrate the different types available. You will need to see what fits with your bike, and consider what configuration and size you might need. The Gobi bags on the left are made by the German company Hepco and Becker, with coloured shock-resistant artificial material that is extremely robust, water-proof and shock absorbing. It combines the look of an aluminium-made bag and the advantages of a bag made of artificial material.
The outer as well as the inner panels of the shell are connected and offer a volume of 3.5 litres for liquids – a novel approach to carrying water or liquids. The bags have a volume of 37 litres and apparently can resist a weight of 100 Kg.
The other luggage carrier often seen is the topbox. These attach behind the pillion seat and again come in hard or soft forms. Hard topboxes are securely attached to the bike frame and lids are lockable so are quite secure. Although good quality topboxes are waterproof and can detach quickly, they are pretty expensive. Anything behind you when riding needs double checking… make sure you check that the topbox has snapped securely into place, as they tend to have a habit of coming adrift.
Tank bags as above can be useful if you use a bike most days, and even OK for a weekend spin if you are staying in accommodation. You can leave the motorcycle panniers at home and just take the tank bag. The bag sits atop the petrol tank and can be attached securely by straps, magnetized base, or even stretch cords. A well designed tank bag generally has a waterproof cover, is expandable, has carry handles, money pockets, a transparent pocket on top used for map reading, soft base so as not to scratch paintwork, and some of them will convert into a backpack.
Tool sets are highly recommended for long journeys, and you should carry all commonly used socket sizes, including wrenches, pliers, ratchets, tire gauge, screwdrivers, and any fuses the bike takes. You can put these in your motorcycle panniers, although I carry my kit in a fork bag, as it is easier to get at. However, if you are travelling in built up areas and where security might be a problem, this is not a good idea.
Fix-a-flat or similar is also useful in the event you have a flat miles from anywhere. But see what happens if the can is not protected properly from thumping over corrugated dirt roads… foam everywhere!