A travel pack is a panel- or front-loading backpack with a long U-shaped zipper running all the way around the top and sides.
It's generally wider and less streamlined than a top-loading backpack, but is probably a better all-purpose choice for most around the world travel.
On these packs, the front panel pretty much opens up the whole main compartment so you can easily get to your gear without having to rummage around for it.
Front-loaders often come with a removable day pack as well, and can be transformed
into a suitcase-style bag by zipping up the shoulder straps and hip belt into a flap at the back or by taking the shoulder harness off altogether.
And that's actually a lot bigger bonus than it sounds!
So, the downside here? Depends on what kinds of activities you think you might be using your backpack for, really.
Even some of these pricier front-loading packs aren't as high-tech as top-loaders built for serious hikers, climbers and wilderness campers. They tend to be heavier, use simpler strap fittings and have less sophisticated back support.
There are some pretty impressive exceptions out there though. So unless you're going to be doing a lot of off-trail or long-distance trekking along the way, a good quality front-loader is a decent choice.
If you decide to go for a front-loading pack, you should look for one with a clip-on carrying strap as well as top and side handles, and a full-length flap at the back that zips up to cover the shoulder straps and hip belt.
Because straps have a nasty habit of getting tangled up with other straps, caught in doorways and snagged on airport conveyor belts.
As most airlines won't compensate you for a damaged backpack (I've tried), it's good to be able to tuck the straps away neatly before checking your bag in at the airport.
Also, if you find yourself in a swanky hotel (hey — it happens!) where the receptionist is giving you snooty looks, you'll come across as a lot more presentable walking in with what looks like a soft-sided suitcase than with a dusty old backpack slinging off your shoulder.
You might find a travel pack with detachable shoulder straps as well, and those do come in handy at times. But if every time you need to convert your pack you have to fiddle with the straps and try to figure out where to stuff them, the whole thing becomes quite a hassle.
It's much easier to just make the straps disappear into the back of the pack with one quick swoop of a zipper.