Enormous efforts have been made to manipulate the light-matter interactions, especially in sub-diffraction-limited space, leading to miniaturized and integrated photonic devices. In physics, an elementary excitation, called polariton, which is the quantum of the coupled photon and polar elementary excitation wave field, underlies the light-matter interaction. In the dispersion relation, polaritons behave as anti-crossing interacting resonance. Surface polaritons provide ultra-confinement of electromagnetic field at the interface, opening up possibilities for sub-diffraction-limited devices, and various field enhancement effects. In the electromagnetic spectra, terahertz (THz) regime was called THz gap before the 1990s, but has now been thrust into the limelight with great significance. This review is devoted to the emerging but rapidly developing field of sub-diffraction-limited THz photonics, with an emphasis on the materials and the physics of surface polaritons. A large breadth of different flavours of materials and surface polaritonic modes have been summarized. The former includes metallic, dielectric, semiconductor, two-dimensional (2D) materials, metamaterials, etc.; the latter covers surface phonon-, plasmon-, and hybrid polaritons. In the THz regime, 2D surface plasmon polariton and artificial surface phonon polaritons offer more attractive advantages in ability to obtain low-loss, tunable, ultracompact light-matter modes.