Editor's note - This post came out many years ago, so many of these specific models are discontinued. See this new post for more updated advice: The Best Backpacks for Travelers.
It's one of the long-term traveler's most important decisions: which backpack should I buy? After all, it is the item you will learn to know most intimately, the vessel holding all of your worldly possessions for an extended period of time. If it's not comfortable and not rugged enough to last, you'll be cursing that backpack every day for a long time as you travel around the globe.
First, dismiss all the backpacks that are meant for hikers and mountain climbers. You do not want a pack that stuffs from the top and is short on pockets. You want one that is meant for constant packing and unpacking, one that will hold all the odd gadgets and small items you need for life on the road. And it needs to be able to lock securely and be thick enough that it won't rip on any stray sharp surface.
Some have wheels (good for a short trip, but not a long one), some are small enough to be carry-ons (great if you can pack that lightly), and some are high-capacity ones suitable for year-long trips that involve cold places. (Pack too much though and it'll be a bitch to carry.)
If you want to make this a popularity contest, then the tried-and-true winners out on the road are travel backpacks from Eagle Creek, like the Grand Voyage 90, and ones from Osprey, like the Waypoint 80. But if the size is right for you, it's also hard to go wrong with North Face or Jansport either.
The specs usually include either cubic inches or cubic liters and you can find conversion charts on the web to figure out equivalencies. The ones linked above are 80 or 90 liters, which is too much for some backs and there's the temptation to fill them up with more than you need. Go smaller if you are only going to be in hot places. I've found the 52-liter Eagle Creek Meridian one to be sufficient if I pack carefully.
Just don't go for some cheapo knock-off to save yourself 50 bucks unless it's a very short backpacking trip you're going on. You'll end up shopping for a new pack by the time you get to Bangkok or Buenos Aires.