So what makes a shrimp and what makes a prawn or jumbo prawn? Both are in the order Decapoda, but there is a special separate suborder of the order Decapoda which contains the prawns (the suborder Dendrobranchiata). There is a different suborder of the order Decapoda for shrimps (the suborder Pleocyemata). However, for most intents and purposes 'shrimp' and 'prawn' are used interchangeably. Shrimps are called prawns. Prawns are called shrimps.
The Decapoda suborder Pleocyemata also contains lobsters, and crabs as well as the 'true' shrimp. The primary distinction is in the structure of the gills which differs from Pleocyemata to Dendrobranchiata. If you wishing to tell a true shrimp from a true prawn, the prawn has the branched gills (with a dendritic structure). The gill structure of the true shrimps is arranged in plates or layers (in a lamellar structure).