Fractons are exotic quasiparticles in 3D quantum systems that cannot move in isolation — they are strictly immobile topological excitations. In models like the X-cube model (Vijay, Haah, Fu 2016), fractons live at corners of membrane operators, while lineons can move only along lines. This restricted mobility leads to sub-extensive ground state degeneracy scaling as L² (compared to topological order's O(1)), ultra-slow thermalization, and connections to fractal structures. Fractons may underpin glassy quantum dynamics and have applications in quantum error correction.