Original: The Malacostraca (Greek: "soft shell") are the largest class of crustaceans and include most of the animals that non-experts recognize as crustaceans, including decapods (such as crabs, lobsters and shrimp), stomatopods (mantis shrimp) and euphausiids (krill).
Simplified: The Malacostraca (Greek: "soft shell") are the largest subgroup of crustaceans. They include decapods (such as crabs,  lobsters and shrimp), stomatopods (mantis shrimp) and krill.