A mineral is an inorganic solid with a definite (fixed) composition and an orderly arrangement of fundamental components (or atoms) called a crystal structure.
Minerals are inorganic, which means that they are without life. Note, however, that minerals may originate from organic processes (e.g. the growth of shells and corals).
Most, but not all rocks are composites of minerals. "Glassy" rocks such as obsidian have no regular arrangements of atoms and thus are not composed of minerals.
The study of minerals is called Mineralogy, and is a fundamental expertise needed for geologic and environmental studies. It is one of the first courses in any Geoscience degree program.
You can view samples of minerals in the display case on the southeast corner of the second floor of Macelwane Hall.