What are network collisions and how do they occur? Are they normal?

A network collision occurs when two devices attempt to transmit data on a shared circuit. In a shared circuit where multiple devices are listening, only a single device can transmit data at a single point in time which requires the rest of the devices to be listening to the transmission. In an hub environments, multiple devices are connected to a single transmission stream, so collisions tend to happen. Switches break up collision domains to have single devices on their own network segment, which eliminates collisions.