Under not too extreme conditions we distinguish just three different states of matter. gas, liquid and solid. Sometimes we denote gas and liquids as fluids. There are important qualitative distinctions between solids and fluids in that a solid can support shear stress while a fluid cannot. What this basically means is that I can make a rod of a solid and hold it from one end while this is certainly not possible for a liquid. Closely related to this facts is the fact that in a solid each atom has a certain average position that it moves around and from which it never strays very far. In a fluid on the other hand atoms move macroscopical distances. Fluids therefore shape themselves according to their surroundings, ie. you can pour fluids and they take the shape of the container they are poured into.
The distinction between a gas and a liquid is quantitative
in that the density of a liquid, ie. its mass per unit volume, is much
greater, typically a factor 
, than for gases. For example the
density of water is about 
kg/m
while the density of air is
1.3 kg/m. The higher density indicates that the atoms are closer in a
liquid than in a gas. If we denote the linear dimensions of cube
which on average contains a single atom by L, then
Thus because the density of liquids are three orders of magnitude larger than
in a gas the atomic spacings are a typically a factor 10 smaller in liquids.
For example in water we have
Where I have introduced the new unit for length 
m.
For air which is mostly nitrogen the characteristic atomic spacing is
much larger:
The effective dimensions of an atom defined as the
range over which it interacts with other atoms
is of order 1-5 Åso we see that
interactions between atoms should be significant in liquids but
much less so in a gas. A result of this is that
while gases can typically be compressed by a large amount
and their density is strongly temperature dependent
this is not so for liquids where the atoms are already packed
about as close as they want to get
.