Solids, liquids, and gases have very contrasting properties. These contrasting properties are explained by the underlying, particle-level characteristics. One of the most important particle-level characteristics is the intermolecular forces acting between adjacent particles. These intermolecular forces are what holds the particles in their place (in solids) in close proximity to one another. The stronger the intermolecular forces are, the more rigid (less movement under the influence of an external force) that the material will be. Strong intermolecular forces also cause a sample of matter to exhibit less flow; when tipped or bumped, the material will not splatter nor pour. When the intermolecular forces are strong, a material will more likely condense together. When placed in a container, it won't spread out throughout the whole volume of the container but instead will maintain its shape. We often say that it has a fixed shape. Finally, a state of matter with strong intermolecular forces will be very difficult to compress into a smaller volume; the forces that hold particles together are strong enough to resist any external pressures intended to compress the sample. We call such states of matter incompressible.
On the other hand, states of matter with weak or no intermolecular forces are not rigid, exhibit the tendency to readily flow, do not have a fixed shape or volume but rather fill the container they are in, and are highly compressible. At the particle level, such states are characterized by particles that do not have a fixed position but rather move about the entire volume of the container at high speeds.
Solids are observed to have the strongest intermolecular forces and gases are observed to have the weakest intermolecular forces. Liquids are the intermediates between solids and gases, both in terms of their intermolecular forces and their observable properties.