Matter

Matter is anything that occupies space and has mass. Mass is the quantity of matter with a unit in grams. There are three states of matter: solid, liquid, and gas. Solid has definite size and shape; liquid has definite volume but takes the shape of a container; and gas has no definite shape and volume.

As for the composition of matter, there are two types -- substances and mixtures. Substances can further be subdivided into two groups, they are element and compound. Element is a substance made up of only one kind of atom. When two or more different atoms join together, the resulting substance is called a compound. An example of compound is water, with two hydrogens and one oxygen. Both of these groups of substances are homogeneous, meaning all parts are uniform.
Mixtures, on the other hand, are varied in their composition. If the composition of mixture is not uniform, then the mixture is considered heterogeneous. If the composition is uniform, then the mixture is considered homogeneous.