A form of matter is another name for the state of matter.
There are three states of matter- gas, liquid, and solid. The state of matter is due to the amount of kinetic energy amongst the atoms or molecules that create the substance. Kinetic energy is the energy of movement.
Gases have the most kinetic energy. Thus, the molecules move fast and far apart. This results in there being a lot of empty space between the particles of gases. Thus, there are not many significant intermolecular forces between the particles of a gas. For this reason, gases are considered fluid (meaning the particles can slide past one another). Gases are said to have no volume or shape of their own.
Liquids have medium amounts of kinetic energy. Thus, the particles of liquids are closer than those of gases and intermolecular forces can exist. Liquids have volume but take the shape of the container in which they are placed.
Solids have little kinetic energy. The molecules are close to one another. Although still moving slightly, the particles of solids are considered to be in fixed position. This allows or strong intermolecular forces. Gases have definite volume and definite shape.