When packing for traveling around the world there are several styles of backpack to chose from.  You can get external frame packs, travel packs, backpacking packs, internal frame packs and frameless packs. If you are planning on hiking for most of your trip, this probably isn’t the article for you. I will be focusing on backpacks for world travelers, who will be exploring urban and rural settings alike. The criteria for my ideal travel pack is one that provides support for light hiking, but has easy access to all of your belongings quickly. A travel backpack should be light, portable, and waterproof (or able to be waterproofed) and hold all of your gear, providing greater mobility.
Backpack buying guide
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Type of Travel Backack:
For most people who need a backpack for a trip around the world the type that I would recommend is an internal frame travel pack. A internal frame travel pack is a specific kind of backpack that serves many of the functions of traditional luggage while being portable like a backpack. Most good travel packs look like backpacks that have a zipper around the edge in the manner of a regular duffel bag, which gives you the ease of packing and allows you to take out specific items without unpacking entirely. While even good travel packs don't offer the same kind of support as hiking backpacks, you aren't packing for the kind of travel where you will be walking 15 miles a day either!
Backpack Extras:
When buying a good travel pack, look for one with covers that conceal the shoulder straps (great for airplanes or buses to prevent tearing of vital straps). Many high quality travel packs also have a zip-off day pack component that can either provide extra space in the bag during travel while quickly and easily convert to a handy bag when just walking around a city. Many of these have a padded laptop sleve as well which is great if you are planning on bringing your laptop traveling.
Backpacks with Wheels
If you will be traveling more in urban areas, you may want to consider buying a travel pack with wheels. Consider where you are planning on traveling on your year long trip; while they add convenience, buying a travel pack with wheels will also add pounds. If you will be in areas with rougher roads or no roads at all, the wheels aren’t going to help much and will only slow you down.
Buy travel backpack
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Whatever backpack or travel pack you buy make sure it has a good return policy. Many quality companies, such as REI or LL Bean have a no questions asked return policy. Buy a bag online in advance of your trip, try walking around with it for a while, and if it doesn't work out, trade it for a new one. I cannot stress enough that you should get the pack well beforehand and test it. Pack it full of everything you are going to take; pack it full with more things that you are going to take and then carry it around with you for a few day. Go to the park, go shopping, bring it to a restaurant. If you plan on doing these thing while you travel (which you will!) this will give you a good sense of if it is working for you.
Osprey Eagle Creek Backpack
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Buy Your Bag:
I personally use an Eagle Creek  Osprey pack from REI which has seen me successfully through my cheap backpacking in many countries and five continents! Every round the world trip is different, and will require a different pack. Backpacks and travel packs range from being relatively cheap to quite expensive, but you will find something that works for any budget. Remember, a good pack will last you almost forever, and retailers like REI and LL Bean usually offer lifetime guarantees that are really worthwhile. I had a friend return a tent to LL Bean that bear had ripped apart (he wasn't in it) and they replaced it for free!
Two more thoughts to bear in mind:
Rain Cover: After you buy your travel pack for backpacking around the world, you are going to want to make sure you have rain protection. Some packs have a built in rain cover, but if not you can easily find a cover on Amazon or the manufacturer’s website.
Lock: You will also want to buy a lock for your backpack to protect against theft. The best locks for backpacks will be able to shut the bag as well as attach it to another object such as a lamppost or bike rack. Make sure to always keep an eye on your backpack while traveling the world! Some backpacks for travel are made to incorporate a lock. 
Further Reading
If you are interested on more ways to pack light read:
For more tips on traveling the world on a budget, read 5 Musts for Cheap World Travel: See the World for Free (Almost!)