As an emerging magnetic phase distinct from traditional ferromagnetism and antiferromagnetism, altermagnetism has garnered significant attention due to its combining zero net magnetization with momentum-dependent spin splitting. This paper provides a comprehensive analysis of the field based on spin-group symmetry theory to elucidate the unique electronic structure in which time-reversal symmetry breaking and crystal rotation operations occur, rather than translation or inverse, enhancing Kramer degeneracy without the need for relativistic spin-orbit coupling. The experimental aspects are surveyed using representative systems such as MnTe, CrSb, and RuO₂. The progress of growing high-quality films through molecular beam epitaxy (MBE) and magnetron sputtering is described in detail, as well as the direct visualization of giant band splitting and domain textures by using angle-resolved photoemission spectroscopy (ARPES) and photoemission electron microscopy (PEEM). Furthermore, the mechanism for manipulating Néel vector is systematically examined, with a focus on strategies involving current-induced spin-splitting torque (SST), lattice strain engineering, and thermal modulation, enabling efficient readout and writing of magnetic states. In addition to the basic properties, the discussion extends to the frontier of material exploration, including metal, two-dimensional and superconducting altermagnets, and evaluation of their transformative potential in high-density magnetic random-access memory (MRAM), topological spintronics, and ultrafast magnonics. The review ultimately identifies the current challenges in material expansion and interface engineering, outlining a roadmap for transitioning altermagnets from fundamental discoveries to practical applications in the next-generation information technology.