Software development is the programming aspect of development such as creating applications for smart phones or more complicated development such as embedded development that controls products.
Electrical engineering allows us to create circuits and boards that control a product. Various sensors and motors might plug into those control boards which actuate the mechanically designed hardware.
Product development encompasses an end to end development strategy that begins with napkin drawings and ends with either working prototypes or a producible product.
Mechanical designers create the physical enclosures, buttons, actuators and housings that a user interacts with. Mechanical designers are also challenged with usability and the aesthetics of the product.