Bags are just pockets you carry. Or maybe pockets are bags sewn into your clothes. Either way, some bags are more like crates or boxes than luggage or purses. The duffel bag epitomizes this idea.
Best suited for transporting a large amount of gear from points A to B where it will be unloaded, the duffel bag is the cargo container of the luggage world. The most basic type is the Sea Bag, made of cotton canvas with an opening on one end and a simple grommet-and-snap-hook closure on the end of the single strap. The majority of duffels on the market have a zippered opening for access and two straps that combine at a handle. They are rectangular or barrel-shaped and built of durable materials, with few internal features.
When you start looking for 'just a duffel bag,' you will be met with a dizzying array of choices. Some key things to look for:
Durability - Look for a tag identifying the use of Cordura nylon, widely recognized as the most durable fabric available. At the least you want the bottom of the bag to survive being dragged across a parking lot or two.
Water resistance - The body fabric should be shiny on the inside. This indicates a polyurethane coating that will keep water from penetrating the whole cloth. Unless all the seams are sealed, however, the bag is not waterproof. Some brands will design the bottom so to reduce stitching and this can greatly increase the protection offered when your bag is dropped in a puddle. Reinforced vinyl is a popular and durable option, but is shiny, heavy and will leak at the seams like a conventional bag.
Waterproof - If the bag has welded-seam construction, so that there are few or no stitch holes, it's considered a waterproof bag and can safely be transported in open trucks, boats, or left out in the rain. These bags will usually have a roll-top closure or very stiff water-tight zippers, which can be tricky to manipulate. Either way, your gear will stay dry, but the ease of use is pretty low.
Longevity - Compare the size and weight of the straps and hardware. Chunky webbing, zippers, and d-rings are stronger, easier to work with, and indicate a bag built to last. Cheaper duffels are frequently outfitted with thinner fabric and less-substantial findings, and though the fabric might hold up, if the straps hurt your hands or the zipper brings out your 'adult' language, it's just not worth it.
Ease of Use - The least expensive and simple duffels will have a zipper opening running straight from end to end, which reduces the amount of stitching, but can be annoying to root around in. If you will be using the bag for storage and/or more like luggage, look for a u- or d-shaped zipper opening that stays open, making packing easier and finding just what you're looking for much faster.
Size - Most duffels will be described by their cargo volume, in either cubic inches or liters. As a reference, school bookbags are in the 30-38L range, and anything under 45L should work as a carry-on. The duffel shape is available in pretty much any size, and it's easy to put just 80 liters of stuff in a 100 liter bag, but will you stop there? How much will it weigh when full? Do you really need that much bag? 'I've got room for that' might lead to a hernia.
Features - Some duffels are more like suitcases, with rolling designs, collapsible handles, frames, and expansion panels added to their strong-but-simple design. All of these will add weight and can possibly need repair down the road, but can also make an 'OK' bag into a 'must have.' Look for removable or concealed shoulder straps, freeing up your hands for tickets, doors, or phone. Dig around in the bag, opening every zipper and pocket.
It's all up to you, but as long as you are clear about defining your needs, you'll end up with the right bag for the job. Whatever your decision, keep in mind that the value of quality will remain long after the satisfaction of savings fades.