We accidentally used Ziploc "storage bags" instead of "freezer bags" to package some meat for the freezer. We noticed this after the fact and it got me thinking: what is the difference? Marketing aside, they both appear to be resealable zipper-like air-tight and water-tight plastic bags.
Freezer bags are slightly thicker plastic, and more forgiving of sharp or pointy foods (like meat with protruding bones).
I know some freezer bags include a white place to write a label on, I don't remember if storage bags do as well.
The press-to-seal bags do a better job of not spilling if accidentally dropped compared to the zipper versions.
A freezer bag actually has an additive that is added to the poly while the film is blown. This helps prevent the plastic from becoming brittle and breaking down while frozen for a long period of time. If you plan on freezing something for a long period of time do not use a regular storage bag.
Thicker plastic does not have as much to do with it as people think. Another additive is added to the poly to make the film stronger and have more stretch to it. Certain recipes for poly will prevent a bag from being a thinner mil thickness than others.
Write on white ink (the white block) on the poly bag is used for many types of bags, auto shops for parts, food storage and so on. For every type of use for a poly bag, packaging salt for sidewalks, fertilizer, food storage, anti-stat, even garbage bags all have different additives used when the plastic is first made.
Freezer bags are thicker and are supposed to slow down the freezing on the ouside so that you don't get so many ice crystals on the outside of the food.
If I run out of freezer bags, I use thin sandwich bags but wrap them in bubble wrap before putting them in the freezer. This also slows down the freezing and prevents ice crystals forming.