"Flexible packaging" gets used as a catch-all term for anything that isn't a rigid bottle, jar, or box. That is a reasonable starting point, but it glosses over what actually makes a package flexible, and why that property matters for cost, performance, and sustainability.
The defining trait: the package changes shape with its contents
A rigid container holds its shape whether it is full or empty. A flexible package, by contrast, conforms to whatever is inside it. A stand-up pouch full of coffee looks different empty than full. A roll of poly bags has no shape at all until something is loaded into one. That conformability is the entire point: less material moves more product, and the package collapses to near nothing once it is empty.
Film, not molded plastic, is the building block
Rigid packaging starts as resin that gets injected or blow-molded into a fixed form. Flexible packaging starts as extruded film, typically blown film, that gets converted into bags, pouches, liners, or rollstock. The manufacturing process is fundamentally different, which is why a custom film extruder and a rigid bottle manufacturer are rarely the same vendor.
Single material vs. structure
Some flexible packaging is a single resin doing every job: containing, sealing, and protecting. A lot of higher-performance flexible packaging is a structure, multiple layers of different materials coextruded or laminated together so that one layer handles strength, another handles barrier, and another handles seal performance. Knowing whether your application needs a single material or a structure is one of the first specification decisions, and it depends entirely on what the package has to survive and protect against.
Why the category keeps growing
Flexible packaging keeps taking share from rigid packaging because it generally uses less material per unit of product protected, ships lighter, and stores flatter before use. None of that means it is automatically the right choice for every product. It means the tradeoffs are different, and they are worth understanding before you assume a switch from rigid to flexible is a simple swap.
Where to start if you are new to specifying film
Start with what the package has to survive, not with a resin name. Define the abuse it will see in shipping, the shelf life it needs to support, and the seal performance the application demands. The right film structure follows from those answers; it should not be chosen first and then forced to fit.