Solids, liquids and gases are the three main phases of matter. Solids have a fixed crystalline structure, whereas liquids and gases are more free-flowing. The phase of matter that a given substance exhibits depends on temperature and pressure.
Solids have the least internal energy of the three states of matter and are typically the coolest. The molecules of a solid are arranged in a fixed pattern independent of the surrounding area. An example of a solid is an ice cube.
Liquids do not have a fixed structure and have slightly more space between their molecules. They adapt their molecular structure to match the container in which they are stored and are subject to gravity. Liquids have a specific viscosity, or ease at which they flow. Water is one of the least viscous liquids, as water molecules move freely. When matter is in liquid form, it has more thermal energy than it does in solid form.
Gases have even more thermal energy and freedom of movement than liquids; in fact, they occupy the entire container in which they are stored. Steam is the gaseous form of liquid water. Not all compounds exhibit all three states of matter; carbon dioxide, for instance, transitions between solid and gaseous form. This is how it came to be called dry ice.