Minerals are the fundamental building-blocks of a rock. A rock is simply a combination of minerals. Minerals are different from a "rock" because a mineral has a specific chemical formula. Rocks can be made of many different types of minerals.

A "mineral" is defined as a naturally-occurring, inorganic solid with a specific chemical composition and a definite crystal structure.

Let's define those terms:
NATURALLY-OCCURRING = minerals are formed in nature by natural processes;
INORGANIC = minerals are not organic compounds, meaning they are not plants or other living things;
SOLID = minerals are solid phases (not liquid and not gas);
SPECIFIC CHEMICAL COMPOSITION = the chemical formula (or list of elements) of a mineral is unique to that mineral, which means every mineral has its own special chemical formula;
CRYSTAL STRUCTUR = the atoms in a mineral are arranged in a defined pattern, which controls the shape of the mineral crystals (for example, the mineral "halite" has a cube pattern for its crystal structure).

There are thousands of unique minerals, and each is made of its own group of elements (atoms in the periodic table). For example, quartz is made of repeating groups of two oxygen atoms and one silicon atom and has the chemical formula SiO₂. Potassium-feldspar (commonly known as K-spar) has the formula KAlSi₃O₈ (one potassium atom, one aluminum atom, three silicon atoms, and eight oxygen atoms).