What Makes a Gas... different?
from the other phases
The three phases of matter differ primarily in the way in which all the molecules arrange themselves relative to each other and how much translational motion there is. Let's knock it down to this:
solid
Solids - the molecules are held rather tightly in position relative to each other. You might even say that the molecules are touching because they are very very close to each other. Solids are a condensed phase of matter. The intermolecular forces (IMFs) for solid state are strong enough to keep the molecules in a static position relative to their neighbor molecules. Many solids have perfect rows and columns (a symmetric lattice) as well. That gets you a crystalline solid. If they don't have perfect rows and columns (no lattice) you get an amorphous solid. See the figure to the right? The solid state is depicted with nice ordering and very close neighbor molecules. A solid doesn't take on the shape of its container either.
liquid
Liquids - liquids are just like solids in that their molecules are very very close to each other (touching). The difference is that liquid molecules tend to have much more energy in the form of translational motion. That means that the particles move all over the place with respect to each other, but still stay together in the condensed phase. A static figure drawing really doesn't do the model justice. We need movement of the molecules. We generally show the phase with the molecules (the little green circles in our diagram) somewhat all out of order and maybe even some little lines that convey movement. Liquids will take on the shape of the container they are in, but are subject to gravity and will fill from the bottom up.
gas
Gases - gases are not condensed phases at all, they are the opposite. None of the molecules in a gas want to stick (and condense) to any of the other gas molecules. The reason is that the intermolecular forces are so small compared to how fast the particle is moving (kinetic energy), there is no sticking at all. As a matter of fact, gases can be very accurately modeled by assuming there is no force at all of attraction between the molecules. This model is called the ideal gas model. Gas molecules spread out and continue to do so until they encounter a wall of their container. Gases will always completely fill the container that they are in - top to bottom, side to side. And, if there is a hole or a lid opened to the container, then the gas molecules will readily start diffusing out into the open space - like in our diagram to the right, molecules are escaping the container because there is no lid to seal them in.
Densities of Solids, Liquids, and Gases
Solids and Liquids are both condensed phases and have densities to match. The symbol for density is usually a greek rho (\(\rho\)) or just a lowercase letter \(d\). A common reference point for many things is water. Water in liquid form has a density of about 1.00 gram per milliliter (g/mL). Any solid that sinks in water has a density greater than 1 g/mL, and if it floats its density is less than 1 g/mL. The densest substance here on earth under normal conditions is the element osmium which is 22 g/mL. You might be more familiar with gold though, its density is 19.3 g/mL.
Gases are much less dense than that of solids and liquids. The air you are breathing is about 0.0012 g/mL which is about 1000× less dense than water. Of course the thing with gases is that they have a very wide range of densities (unlike liquids and solids). The higher the pressure, the more dense the gas. The lower the temperature, the more dense the gas. So air is at its most dense in cold temperatures and relatively high pressures - which basically means at sea level in altitude.
Hopefully this has given you a pretty good idea about the 3 states of matter. There is certainly more to learn and you should feel free to read more online from other sources.
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