A phase is another term for a state of matter. There are four phases - solids, liquids, gases, and plasma.
Solid[]
A solid has definite volume, shape and mass, and are very hard to compress. Solids matter possesses the highest density in most cases because its molecules are closely packed together with no wiggle room. Therefore, if the molecules gain heat and energy, the gaps between them will expand and the density will often decrease. Solid is abbreviated as "s". For example, ice is written as H2O(s).
Liquid[]
Liquids have definite volume and mass. Liquid molecules are more spread apart than solids with enough room to move up to a certain extent. As a result, liquids take the shape of any container they fill. As temperature increases, the molecules move faster and become farther away from each other, and liquids can turn into gas when it reaches the boiling point. Liquids also constantly evaporates into gases. The only two known elements that are liquid at room temperature are mercury and bromine. Liquid is abbreviated as "l". For example, liquid water is written as H2O(g).
Aqueous[]
Aqueous substances are dissolved in water. For example, aqueous sodium chloride is sodium chloride dissolved in water. "Aqueous" is technically not a phase of matter, but sometimes aqueous substances are specially labelled in chemical formulas. For example, aqueous hydrogen chloride is written as HCl(aq).
Gas[]
Gas does not possess a definite shape or volume, since gas molecules move very quickly. The density of gases are usually significantly smaller than solids and liquids. Gases can usually be compressed easily. Cooling a gas to temperatures lower than its boiling point or increasing the pressure to a certain extent can turn it into a liquid. Examples of gases include hydrogen and helium. Gas is abbreviated as "g". For example, carbon dioxide gas is written as CO2(g).
Plasma[]
Plasma is formed when a gas is heated. The molecules become excited and move at a much faster pace, causing atoms to collide and shaking electrons loose. Therefore, plasma usually contains mostly charged particles like ions and electrons. This state of matter is not very common in chemical reactions, but is common in the universe. For example, the sun is mostly plasma.