Matter exists in three forms called three states of matter. They are solid, liquid and gas.
Solids: A stone, a table or a book are solids. They are hard to touch. Suppose, if we pick them up and place them somewhere else, their shape or the space they occupy does not change. Solids therefore, have a definite shape and volume.
In solids, the molecules are very close to each other. So we can say that the molecules of a solid are closely packed.
Liquids: Water, milk and mango juice are liquids. We cannot pick them up with our fingers. They flow if we pour them on the floor. Pour a liquid in a container such as a glass. Now we see the shape of the liquid is the same as that of the glass. Now pour it in a bowl and we see that it take the shape of the bowl. But the volume of the liquid does not change no matter which container it is put in.
So a liquid has a definite volume but no definite shape. In liquid, they are less closely packed in comparison to solids. In case of liquid, they can move around more freely and that is why liquids can flow.
Gases: Air is a gas. Molecules in a gas are far apart from each other. They are free to move around all over in the space available to them. So a gas has no fixed shape or volume. Molecules of a gas can be squeezed into a small container or spread out in a big container. In gases, the molecules are very loosely packed.
Suppose, when we blow air into a balloon, the air is squeezed into the balloon. When we let the air out of the balloon, it spread all over the room. It now occupies a large volume. So gases have no fixed shape or volume.
Due to arrangements of molecules, solid have a definite shape and volume. Liquids have a definite volume but no definite shape. And gases have neither definite volume nor shape.