Optical fibers have evolved from light-guiding structures into versatile sensing media. The fundamental aspects of optical fibers are reviewed, including their structure, light propagation, and classification, together with key transmission characteristics such as attenuation, scattering, and dispersion. Attention is given to distributed optical fiber sensing, in which Rayleigh, Brillouin and Raman scattering mechanisms enable the measurement of physical parameters such as temperature, strain, and dynamic perturbations along the fiber. The paper further emphasizes the relationship between propagation phenomena and sensing performance, providing a structured perspective that evolves from optical fiber fundamentals to sensing technologies.