Long-range ordering of dipoles is a key microscopic signature of ferroelectrics. These ordered dipoles form ferroelectric domains, which can be reoriented by electric fields. Relaxor ferroelectrics are a type of ferroelectric where the long-range ordering of dipoles is disrupted by cation disorder, exhibiting complex polar states with a significant amount of local structural heterogeneity at the nanoscale. They are the materials of choice for numerous devices such as capacitors, nonlinear optical devices, and piezoelectric transducers, owing to their extraordinary dielectric, electro-optic, and electromechanical properties. However, despite their extensive applications in these devices, the origins of their unique properties are yet to be fully understood, hindering the design and exploration of new relaxor ferroelectric-based materials. Herein, the complex polar states and applications of relaxor ferroelectrics are first introduced. Attention is then focused on their electromechanical properties, where the relationship between local structural heterogeneity and the extraordinary electromechanical properties is discussed. Based on the understanding of relaxor ferroelectrics, potential strategies to exploit the local structural heterogeneity to design ferroelectrics for drastically enhancing their electromechanical performances are also discussed. It is expected that this article will stimulate future studies on the important roles of local structural heterogeneity in improving the properties of various functional materials.