Fermions are a class of subatomic particles that follow the Pauli exclusion principle, which states that no two identical fermions can occupy the same quantum state simultaneously. This property makes them essential for forming matter, as they include particles like electrons, protons, and neutrons. Fermions have half-integer spins, such as 1/2 or 3/2, which distinguishes them from another class of particles called bosons. Fermions are responsible for the structure of atoms and the behavior of matter at low temperatures. They can be found in various states, including solid, liquid, and gas. Their interactions and arrangements determine the physical properties of materials, making them fundamental to the study of quantum mechanics and particle physics.