Single-mode fibre uses a thin core to guide light in a single path, enabling long-distance, high-bandwidth transmissions. Multi-mode fibre, on the other hand, has a thicker core that allows multiple light paths to travel simultaneously. While this can increase bandwidth over shorter distances, it can also lead to signal distortion due to the different light paths arriving at different times. This makes multi-mode fibre more suitable for shorter-distance applications, while single-mode fibre is better suited for longer-distance transmissions.