(small) bag [noun, diminutive] [het zakje, de zakjes]

This word is used to indicate small paper or plastic bags, usually without handles and pretty much shapeless.

A "tasje" is also a small plastic bag, but has handles and is shaped like a plastic bag.

Examples:
1. "Boterhamzakje": sandwhich bag.
2. "Suikerzakje": sugar bag.
3. "Vuilniszak": garbage bag.
4. <shop attendant:> "Wilt u een tasje?"
("Would you like a bag?")

Caution: "Zak" is also a term of abuse, indicating a certain part of a certain male body part.

Extra:
Check out this British comedian joking about the Albert Heijn supermarket and the concept of "zakje".
It takes 7 minutes of your lunch break and is, although a bit out-dated, highly recommended.