A bag is a paper or plastic container that something is sold in.
I bought a bag of crisps and a drink.
They sell herbs in plastic bags.
A bag of something can refer either to a bag and its contents, or just to the contents.
She bought a bag of flour.
He ate a whole bag of sweets.
A bag is also a soft container that you use to carry things in.
Mia put the shopping bags on the kitchen table.
You can call a woman's handbag her bag.
She opened her bag and took out her keys.
You can call someone's luggage their bags.
They went to their hotel room and unpacked their bags.
A single piece of luggage is a case or suitcase.
The driver helped me with my case.
She was carrying a heavy suitcase.